UC-NftLF 


III  III  II 11  III  II 

•C    107    SIS 


«*.-••  •  .*• 
••: 


l>5- 


THE 


CORRELATION  AND  CONSERVATION 

OF 

FOECES: 

0f 


PEOF.  GEOYE,  PEOF.  HELMHOLTZ,  DE.  MAYEE, 

DE.  FAEADAY,  PEOF.  LIEBIG  AND 

DE.  CAEPEJSTTEE. 


INTKODUCTION  AND  BRIEF  BIOGRAPHICAL  NOTICES  OF  THE 
CHIEF  PROMOTERS   OF  THE  NEW  VIEWS. 


BT 

EDWARD  L.  YOUMANS,  M.D. 


1 — The  highest  law  in  physical  science  which  our  faculties  permit  ua  to  perceive— the 
Conservation  of  Force.11— DB.  FABADAY. 


NEW  YOEK: 
D.    APPLETON   AND    COMPANY, 

448  &  445  BROADWAY. 
1865. 

LIBRARY  OF  THE 

ninnr  „ 


ENTERED,  according  to  Act  of  Congress,  in  the  year  1864,  by 

D.  APPLETON  AND  COMPANY, 

In  the  Clerk's  Office,  of  the  District  Court  of  the  United  States  for  the 
Southern  District  of  New  York. 


QC 


TO 


JOHN  WILLIAM  DKAPEK,  M.D.,  LL.D, 

PROFESSOR    OF    CHEMISTRY    AND    PHYSIOLOGY    IN    THE 
UNIVERSITY   OP   NEW   YORK. 

DEAR  SIR:— 

IT  seems  peculiarly  appropriate  that  this  volume  should  be  dedicated  to 
you.  Knowing  the  eminent  esteem  in  which  you  are  held  in  the  circles  of 
European  science,  I  cannot '  doubt  that  the  distinguished  authors  of  the  fol- 
lowing essays  would  cordially  approve  this  connection  of  your  name  with 
their  introduction  to  the  American  public. 

There  is,  besides,  a  further  reason  for  this  in  that  large  coincidence  of 
purpose  which  is  manifest  in  their  labors  and  your  own.  For  while  the  per- 
vading design  of  the  present  collection  is  to  widen  the  range  of  thought  by 
unfolding  a  broader  philosophy  of  the  energies  of  nature,  your  own  compre- 
hensive course  of  research — beginning  with  an  extended  series  of  experi- 
mental investigations  in  chemical  physics  and  physiology,  and  rising  to  the 
consideration  of  that  splendid  problem,  the  bearing  of  science  upon  the  His- 
tory of  the  Intellectual  Development  of  Europe — has  powerfully  contributed 
to  the  same  noble  end  ;  that  of  elevating  the  aim  and  enlarging  the  scope  of 
scientific  inquiry. 

I  gladly  avail  myself  of  this  occasion  to  say  how  greatly  I  am  indebted 
to  your  writings,  in  which  accurate  and  profound  instruction  is  so  often  and 
happily  blended  with  the  charms  of  poetic  eloquence.  That  you  may  live 
long  to  enjoy  your  well- won  honors,  and  to  contribute  still  further  to  the 
triumphant  advance  of  scientific  truth,  is  the  heartfelt  wish  of 

Yours  truly, 

E.  L.  Y. 


PREFACE. 


IN  his  address  before  the  British  Association  for  the  Advance- 
ment of  Science  last  year,  the  President  remarked  that  the  new 
views  of  the  Correlation  and  Conservation  of  Forces  constitute  the 
most  important  discovery  of  the  present  century.  The  remark  is 
prohably  just,  prolific  as  has  been  this  period  in  grand  scientific  re- 
sults. No  one  can  glance  through  the  current  scientific  publica- 
tions without  perceiving  that  these  views  are  attracting  the  pro- 
found attention  of  the  most  thoughtful  minds.  The  lively  con- 
troversy that  has  been  carried  on  for  the  last  two  or  three  years 
respecting  the  share  that  different  men  of  different  countries  have 
had  in  their  establishment,  still  further  attests  the  estimate  placed 
upon  them  in  the  scientific  world. 

But  little,  however,  has  been  published  in  this  country  upon  the 
subject;  no  complete  work,  I  believe,  except  the  admirable  volume 
of  Prof.  Tyndall  on  "  Heat  as  a  Mode  of  Motion,"  in  which  the 
new  philosophy  is  adopted,  and  applied  to  the  explanation  of  ther- 
mal phenomena  in  a  very  clear  and  forcible  manner.  I  have,  there- 
fore, thought  it  would  be  a  useful  service  to  the  public  to  reissue 
some  of  the  ablest  presentations  of  these  views  which  have  ap- 
peared in  Europe,  in  a  compact  and  convenient  form.  The  selec- 
tion of  these  discussions  has  been  determined  by  a  desire  to  com- 
bine clearness  of  exposition  with  authority  of  statement.  In  the 
first  of  these  respects  the  essays  will  speak  for  themselves ;  in  re- 
gard to  the  last  I  may  remark  that  all  the  authors  quoted  stand 
high  as  founders  of  the  new  theory  of  forces.  Although  I  am  not 


VI  PREFACE. 

aware  that  Prof.  Liebig  has  made  any  claims  in  this  direction,  yet 
it  can  scarcely  be  doubted  that  his  original  researches  in  Animal 
Chemistry  tended  strongly  toward  the  promotion  of  the  science  of 
vital  dynamics. 

The  work  of  Professor  Grove,  which  is  here  reprinted  in  full, 
has  a  high  European  reputation,  having  passed  to  the  fourth  edi- 
tion in  England,  and  been  translated  into  several  continental  lan- 
guages. It  is  hardly  to  the  credit  of  science  in  our  country,  that 
this  is  the  first  American  edition.  The  eloquent  and  interesting 
paper  of  Helmholtz,  though  delivered  as  a  popular  lecture,  was 
translated  for  the  Philosophical  Magazine,  and  has  been  very  highly 
appreciated  in  scientific  circles.  Th%  three  articles  of  Mayer, 
which  were  also  translated  for  the  Philosophical  Magazine,  will 
have  interest  not  only  because  of  the  great  ability  with  which  the 
subjects  are  treated,  but  as  emanating  from  a  man  who  stands  per- 
haps preeminent  among  the  explorers  in  this  new  tract  of  inquiry. 
The  researches  of  Faraday  in  this  field  have  been  conspicuous  and 
important,  and  his  argument  is  marked  by  the  depth  and  clearness 
which  characterize,  in  an  eminent  degree,  the  writings  of  this  ex- 
traordinary man.  The  essay  of  Liebig  forms  a  chapter  in  the  last 
edition  of  his  invaluable  'Familiar  Letters  on  Chemistry,'  which 
has  not  been  republished  here ;  and,  as  it  touches  the  relation  of  the 
subject  to  organic  processes,  it  forms  a  fit  introduction  to  the  final 
article  of  the  series  by  Dr.  Carpenter,  on  the  "  Correlation  of  the 
Physical  and  Vital  Forces."  The  eminent  English  physiologist  has 
worked  out  this  branch  of  the  subject  independently,  and  the  pa- 
per quoted  gives  evidence  of  being  prepared  with  his  usual  care 
and  ability.  A  certain  amount  of  repetition  is  of  course  unavoida- 
ble in  such  a  collection,  yet  the  reader  will  find  much  less  of  this 
than  he  might  be  inclined  to  look  for,  as  each  writer,  in  elaborating 
the  subject,  has  stamped  it  with  his  own  originality. 

In  the  introduction  I  have  attempted  to  bring  forward  certain 
facts  in  the  history  of  these  discoveries,  in  which  we  as  Ameri- 
cans have  a  special  interest,  and  also  to  indicate  several  applications 
of  the  new  principles  which  are  not  treated  in  the  volume.  It 
seemed  best  to  confine  the  general  discussion  to  those  aspects  of  the 
subject  upon  which  most  thought  had  been  expended,  and  which 
may  be  regarded  as  settled  among  advanced  scientific  men.  But 
there  are  other  applications  of  the  doctrine,  of  the  highest  interest, 
which  though  incomplete  are  yet  certain,  and  these  will  be  found 


PREFACE.  Vll 

briefly  noticed  in  the  introductory  observations — too  briefly,  I  fear, 
to  be  satisfactory.  Those,  however,  who  desire  to  pursue  still 
further  this  branch  of  the  inquiry — the  correlation  of  the  vital, 
mental,  and  social  forces — are  referred  to  the  last  edition  of  Car- 
penter's "  Principles  of  Human  Physiology ;  "  Morell's  "  Outlines 
of  Mental  Philosophy ; "  Laycock's  "  Correlations  of  Consciousness 
and  Organization ;  "  Sir  J.  K.  Shuttle  worth's  address  before  the 
Social  Science  Congress  of  1860,  on  the  "Correlation  of  the  Moral 
and  Physical  Forces;"  Hinton's  "Life  in  Mature,"  and  "First 
Principles  "  of  Herbert  Spencer's  new  system  of  Philosophy.  The 
first  and  last  of  these  works  are  the  only  ones,  it  is  believed,  that 
have  appeared  in  an  American  form,  and  the  last  is  much  the 
ablest  of  all ;  I  was  chiefly  indebted  to  it  in  preparing  the  latter 
part  of  the  introduction.  The  biographical  notices,  brief  and  im- 
perfect as  they  are,  it  is  hoped  may  enhance  the  reader's  interest 
in  the  volume. 

I  have  been  specially  incited  to  procure  the  publication  of  a 
work  of  this  kind,  by  the  same  motive  that  has  impelled  me  to 
write  upon  the  subject  elsewhere ;  a  conviction  of  our  educational 
needs  in  this  direction.  The  treatment  of  a  vast  subject  like  this 
in  ordinary  school  text-books,  is  at  best  quite  too  limited  for  the 
requirements  of  the  active-minded  teacher ;  to  such,  a  volume  like 
the  present  may  prove  invaluable. 

But  a  more  serious  difficulty  is  that,  until  compelled  by  the  de- 
mands of  intelligent  teachers,  the  compilers  of  school-books  will 
pass  new  views  entirely  by,  or  ^ive  them  a  mere  hasty  and  careless 
notice,  while  continuing  to  inculcate  the  old  erroneous  doctrines. 
And  thus  it  is  that  from  inveterate  habit,  or  intellectual  sluggish- 
ness, or  a  shrewd  calculation  of  the  indifference  of  teachers,  out- 
worn and  effete  ideas  continue  to  drag  through  school-books  for 
half  a  century  after  they  have  been  exploded  in  the  world  of  liv- 
ing science.  He  who  continues  to  teach  the  hypothesis  of  caloric, 
falsifies  the  present  truth  of  science  as  absolutely  as  he  would  do 
in  teaching  the  hypothesis  of  phlogiston ;  in  fact,  the  reasons  of- 
fered for  persisting  in  the  erroneous  notions  of  the  materiality  of 
heat — convenience  of  teaching,  unsettledness  of  the  new  vocabu- 
lary, &c.,  are  precisely  those  that  were  offered  for  clinging  to  phlo- 
giston, and  rejecting  the  Lavoiserian  chemistry  of  combustion. 
Both  conceptions  have  no  doubt  been  of  service,  but  both  were 
transitional,  and  having  done  their  work  they  become  hindrances 


Vlll  PREFACE. 

instead  of  helps.  "We  can  now  see  that  when  the  true  chemistry 
of  combustion  was  once  reached,  the  notion  of  phlogiston  was  of 
no  further  use,  and  if  retained  could  only  produce  confusion  and 
prevent  the  reception  of  correct  ideas.  So  with  caloric,  and  those 
false  conceptions  of  the  materiality  of  forces,  which  it  implies :  not 
only  are  they  errors,  hut  the  ideas  they  involve  are  radically  in- 
compatible with  the  higher  truths  to  which  science  has  advanced ; 
so  that  while  the  errors  are  retained  the  truths  cannot  be  received. 

Nor  will  it  answer  merely  to  mention  the  new  views  while 
adopting  the  old,  on  the  plea  that  the  facts  are  the  same  in  both 
cases.  The  facts  are  very  far  from  being  the  same  in  both  cases.  It 
is  precisely  because  the  old  ideas  are  out  of  harmony  with  the  facts, 
and  can  no  longer  correctly  explain  and  express  them,  that  new  ideas 
are  sought.  Was  not  phlogiston  abandoned  because  it  no  longer 
agreed  with  the  facts  ?  So  with  the  conception  of  the  materiality 
of  the  forces ;  it  contradicts  the  facts,  and  therefore,  for  scientific 
purposes,  can  no  longer  represent  them.  In  the  workshop  it  may 
perhaps  be  very  well  to  magnify  facts,  and  depreciate  their  theoreti- 
cal explanations,  but  not  in  the  school-room ;  the  business  is  here  not 
working,  but  thinking.  It  is  the  aim  of  art  to  use  facts,  but  of  sci- 
ence to  understand  them.  And  it  is  simply  because  science  goes 
beyond  the  fact  to  its  explanation,  and  is  ever  striving  after  the 
highest  truth,  that  it  is  fitted  to  discipline  the  thinking  and  reason- 
ing faculties,  and  therefore  has  imperative  educational  claims. 

In  therefore  bringing  forward  these  able  and  authoritative  ex- 
positions in  a  form  readily  accessible  to  teachers,  I  trust  I  am  not 
only  doing  them  a  helpful  service,  but  that  they  will  be  led  to  re- 
quire of  the  preparers  of  school-books  a  more  conscientious  per- 
formance of  their  tasks,  and  that  the  interests  of  sound  education 
will  be  thereby  promoted. 

NBW  YOBK,  Oct.  1, 1864. 


CONTENTS. 


PREFACE,          ...  .        v 

INTRODUCTION,        .  .  .  .  .  .  li 

THE  CORRELATION  OF  PHYSICAL  FORCES,  BY  W.  R.  GROVE. 

Preface,     .......        3 

I. — Introductory  Remarks,       ....  9 

H.— Motion,  .  .  .  .  .  .25 

IE.— Heat,          ......  39 

IV.— Electricity,       .  .  .  .  .  .83 

V.— Light, 110 

VI.— Magnetism,     .  .  .  .  .  .     142 

VH.— Chemical  Affinity,  .  .  .  .  152 

Vin.— Other  Modes  of  Force,  .  .  .  .169 

IX.— Concluding  Remarks,         .  .  .  .1*78 

Notes  and  References,       .....    200 

ON    THE    INTERACTION  OF   NATURAL    FORCES,    BY    PROF. 

HELMHOLTZ,  ,  .  .  .  .211 

REMARKS  ON  THE  FORCES  OF  INORGANIC  NATURE,  By  DR. 

J.  R.  MAYER,    ...  .251 


X  CONTENTS. 

PAGE 

ON  CELESTIAL  DYNAMICS,  BY  J.  R.  MAYER,  .  .          269 

I. — Introduction,  .....     259 

II.— Sources  of  Heat,    .....         261 
HI.— Measure  of  the  Sun's  Heat,     .  .  .  .264 

IV.— Origin  of  the  Sun's  Heat,  .  .  .276 

V. — Constancy  of  the  Sun's  Mass,  .  .  .282 

VI.— The  Spots  on  the  Sun's  Disc,         .  .  .286 

VH.— The  Tidal  Wave, 291 

VIIL— The  Earth's  Interior  Heat,  .  .  .300 

REMARKS  ON  THE  MECHANICAL  EQUIVALENT  OF  HEAT,  By 

DR.  J.  R,  MAYER,         .....    316 

SOME  THOUGHTS  ON  THE  CONSERVATION  OF  FORCE,  BY  Dr. 

FARADAY,    ......          359 

THE  CONNECTION  AND   EQUIVALENCE  OF  FORCES,  BY  PROF. 

LIEBIG,  ......    387 

ON    THE    CORRELATION    OF    THE    PHYSICAL    AND    VITAL 

FORCES,  BY  DR.  CARPENTER,          .  .  .401 

I.— Relation  of  Light  and  Heat  to  the  Vital  Forces  of  Plants,  401 
II.— Relation  of  Light  and  Heat  to  the  Vital  Forces  of  Ani- 
mals,        .  .  .  .  .  .420 


INTRODUCTION 


THERE  are  many  who  deplore  what  they  regard  as  the  material- 
izing tendencies  of  modern  science.  They  maintain  that  this  pro- 
found and  increasing  engrossment  of  the  mind  with  material  ob- 
jects is  fatal  to  all  refining  and  spiritualizing  influence.  The  cor- 
rectness of  this  conclusion  is  open  to  serious  question :  indeed,  the 
history  of  scientific  thought  not  only  fails  to  justify  it,  but  proves 
the  reverse  to  be  true.  It  shows  that  the  tendency  of  this  kind  of 
inquiry  is  ever /row  the  material,  toward  the  abstract,  the  ideal,  the 
spiritual. 

"We  may  appeal  to  the  oldest  and  most  developed  of  the  sciences 
for  confirmation  of  this  statement.  The  earliest  explanations  of 
the  celestial  movements  were  thoroughly  and  grossly  material,  and 
all  astronomic  progress  has  been  toward  more  refined  and  ideal 
views.  The  heavenly  bodies  were  at  first  thought  to  be  supported 
and  carried  round  in  their  courses  by  solid  revolving  crystalline 
spheres  to  which  they  were  attached.  This  notion  was  afterward 
replaced  by  the  more  complex  and  mobile  mechanism  of  epicy- 
cles. To  this  succeeded  the  hypothesis  of  Des  Cartes',  who  rejected 
the  clumsy  mechanical  explanation  of  revolving  wheelwork,  and 
proposed  the  more  subtile  conception  of  ethereal  currents,  which 
constantly  whirled  around  in  vortices,  and  bore  along  the  heavenly 
bodies.  At  length  the  labors  of  astronomers,  terminating  with 


Xll  INTRODUCTION. 

Newton,  struck  away  these  crude  devices,  and  substituted  the  action 
of  a  universal  immaterial  force.  The  course  of  astronomic  science 
has  thus  been  on  a  vast  scale  to  withdraw  attention  from  the  mate- 
rial and  sensible,  and  to  fix  it  upon  the  invisible  and  supersensuous. 
It  has  shown  that  a  pure  principle  forms  the  immaterial  foundation 
of  the  universe.  From  the  baldest  materiality  we  rise  at  last  to  a 
truth  of  the  spiritual  world,  of  so  exalted  an  order  that  it  has  been 
said  Ho  connect  the  mind  of  man  with  the  Spirit  of  God.' 

The  tendency  thus  illustrated  by  astronomy  is  characteristic  in 
a  marked  degree  of  all  modern  science.  Scientific  inquiries  are 
becoming  less  and  less  questions  of  matter,  and  more  and  more 
questions  of  force ;  material  ideas  are  giving  place  to  dynamical 
ideas.  "While  the  great  agencies  of  change  with  which  it  is  the 
business  of  science  to  deal — heat,  light,  electricity,  magnetism,  and 
affinity,  have  been  formerly  regarded  as  kinds  of  matter  '  impon- 
derable elements,'  in  distinction  from  other  material  elements, 
these  notions  must  now  be  regarded  as  outgrown  and  abandoned, 
and  in  their  place  we  have  an  order  of  purely  immaterial  forces. 

Toward  the  close  of  the  last  century  the  human  mind  reached 
the  great  principle  of  the  indestructiblity  of  matter.  "What  the 
intellectual  activity  of  ages  had  failed  to  establish  by  all  the  re- 
sources of  reasoning  and  philosophy,  was  accomplished  by  the  in- 
vention of  a  mechanical  implement,  the  balance  of  Lavoisier. 
When  nature  was  tested  in  the  chemist's  scale-pan,  it  was  first 
found  that  never  an  atom  is  created  or  destroyed ;  that  though 
matter  changes  form  with  protean  facility,  traversing  a  thousand 
cycles  of  change,  vanishing  and  reappearing  incessantly,  yet  it 
never  wears  out  or  lapses  into  nothing. 

The  present  age  will  be  memorable  in  the  history  of  science  for 
having  demonstrated  that  the  same  great  principle  applies  also  to 
forces,  and  for  the  establishment  of  a  new  philosophy  concerning 
their  nature  and  relations.  Heat,  light,  electricity,  and  magnetism 
are  now  no  longer  regarded  as  substantive  and  independent  exist- 
ences—subtile fluids  with  peculiar  properties,  but  simply  as  modes 


THE   NEW  DOCTRINE   OF   FORCES. 


xiii 


of  motion  in  ordinary  matter ;  forms  of  energy  which  are  capable 
of  mutual  conversion.  Heat  is  a  mode  of  energy  manifested  by 
certain  effects.  It  may  be  transformed  into  electricity,  which  is 
another  form  of  force  producing  different  effects.  Or  the  process 
may  be  reversed ;  the  electricity  disappearing  and  the  heat  reap- 
pearing. Again,  mechanical  motion,  which  is  a  motion  of  masses, 
may  be  transformed  into  heat  or  electricity,  which  is  held  to  be  a 
motion  of  the  atoms  of  matter,  while,  by  a  reverse  process,  the  mo- 
tion of  atoms,  that  is,  heat  or  electricity,  may  be  turned  back  again 
into  mechanical  motion.  Thus  a  portion  of  the  heat  generated  in 
a  locomotive  is  converted  into  the  motion  of  the  train,  while  by 
the  application  of  the  brakes  the  motion  of  the  train  is  changed 
back  again  into  the  heat  of  friction. 

These  mutations  are  rigidly  subject  to  the  laws  of  quantity.  A 
given  amount  of  one  force  produces  a  definite  quantity  of  another . 
so  that  power  or  energy,  like  matter,  can  neither  be  created  nor 
destroyed :  though  ever  changing  form,  its  total  quantity  in  the  uni- 
verse remains  constant  and  unalterable.  Every  manifestation  of 
force  must  have  come  from  a  preexisting  equivalent  force,  and  must 
give  rise  to  a  subsequent  and  equal  amount  of  some  other  force. 
"When,  therefore,  a  force  or  effect  appears,  we  are  not  at  liberty  to 
assume  that  it  was  self-originated,  or  came  from  nothing ;  when  it 
disappears  we  are  forbidden  to  conclude  that  it  is  annihilated :  we 
must  search  and  find  whence  it  came  and  whither  it  has  gone ;  that 
is,  what  produced  it  and  what  effect  it  has  itself  produced.  These 
relations  among  the  modes  of  energy  are  currently  known  by  the 
phrases  Correlation  and  Conservation  of  Force. 

The  present  condition  of  the  philosophy  of  forces  is  perfectly 
paralleled  by  that  of  the  philosophy  of  matter  toward  the  close  of 
the  last  century.  So  long  as  it  was  admitted  that  matter  in  its 
various  changes  may  be  created  or  destroyed,  chemical  progress 
was  impossible.  If,  in  his  processes,  a  portion  of  the  material  dis- 
appeared, the  chemist  had  a  ready  explanation — the  matter  was 
destroyed;  his  analysis  was  therefore  worthless.  But  when  he 


XIV  INTEODUCTION. 

started  with  the  axiom  that  matter  is  indestructible,  all  disappear- 
ance of  material  during  his  operations  was  chargeable  to  their  im- 
perfection. He  was  therefore  compelled  to  improve  them — to  ac- 
count in  his  result  for  every  thousandth  of  a  grain  with  which  he 
commenced ;  and  as  a  consequence  of  this  inexorable  condition, 
analytical  chemistry  advanced  to  a  high  perfection,  and  its  conse- 
quences to  the  world  are  incalculable.  Precisely  so  with  the  anal- 
ysis of  forces.  So  long  as  they  are  considered  capable  of  being 
created  and  destroyed,  the  quest  for  them  will  be  careless  and  the 
results  valueless.  But  the  moment  they  are  determined  to  be  in- 
destructible, the  investigator  becomes  bound  to  account  for  them ; 
all  problems  of  power  are  at  once  affected,  and  the  science  of  dy- 
namics enters  upon  a  new  era. 

The  views  here  briefly  stated  will  be  found  fully  and  variously 
elucidated  in  the  essays  of  the  present  volume ;  in  these  introduc- 
tory remarks  I  propose  to  offer  some  observations  on  their  history 
and  the  extended  scope  of  their  application. 

I  have  spoken  of  the  principles  of  Correlation  and  Conservation 
of  Forces  as  established ;  it  may  be  well  to  state  the  sense  in  which 
this  is  to  be  taken.  They  have  been  accepted  by  the  leading  scien- 
tific minds  of  all  nations  with  remarkable  unanimity ;  their  discus- 
sion forms  a  leading  element  in  scientific  literature,  while  they  oc- 
cupy the  thoughts  and  guide  the  investigations  of  the  most  philo- 
sophical inquirers.  But  while  science  holds  securely  her  new  pos- 
session as  a  fundamental  principle,  its  various  phases  are  by  no 
means  completely  worked  out.  Not  only  has  there  been  too  little 
time  for  this,  even  if  the  views  were  far  less  important,  but  the 
questions  started  lie  at  the  foundation  of  all  branches  of  science, 
and  can  only  be  fully  elucidated  as  these  advance  in  their  develop- 
ment. The  new  doctrine  of  forces  is  now  in  much  the  same  con- 
dition as  was  the  new  astronomy  of  Copernicus.  It  is  not  with- 
out its  difficulties,  which  time  alone  must  be  trusted  to  remove ; 
but  it  simplifies  so  many  problems,  clears  up  so  many  obscurities, 


THE   HISTORY   OF   SCIENTIFIC   DISCOVERY.  XV 

opens  so  extended  a  range  of  new  investigations,  and  contrasts  so 
strongly  with  the  complexities  and  incongruities  of  the  older  doc- 
trines, as  to  leave  little  liberty  of  choice  between  the  opposing  theo- 
ries. Not  only  do  the  reception  of  these  views  mark  a  signal  epoch  in 
the  progress  of  science,  but  from  their  comprehensive  bearings  and 
the  luminous  glimpses  which  they  open  into  the  most  elevated  re- 
gions of  speculative  inquiry,  they  have  a  profound  interest  for 
many  thinkers  who  give  little  attention  to  the  specialties  of  exact 
science. 

In  the  history  of  human  affairs  there  is  a  growing  conception 
of  the  action  of  general  causes  in  the  production  of  events,  and  a 
corresponding  conviction  that  the  part  played  by  individuals  has 
been  much  exaggerated,  and  is  far  less  controlling  and  permanent 
than  has  been  hitherto  supposed.  So  also  in  the  history  of  science 
it  is  now  acknowledged  that  the  progress  of  discovery  is  much 
more  independent  of  the  labors  of  particular  persons  than  has  been 
formerly  admitted.  Great  discoveries  belong  not  so  much  to  indi-  \ 
viduals  as  to  humanity ;  they  are  less  inspirations  of  genius  than  j 
births  of  eras.  As  there  has  been  a  definite  intellectual  progress, 
thought  has  necessarily  been  limited  to  the  subjects  successively 
reached.  Many  minds  have  been  thus  occupied  at  the  same  time 
with  similar  ideas,  and  hence  the  simultaneous  discoveries  of  inde- 
pendent inquirers,  of  which  the  history  of  science  is  so  full.  Thus 
at  the  close  of  the  sixteenth  century,  philosophers  had  entered 
upon  the  investigation  of  the  laws  of  motion,  and  accordingly  we 
find  Galileo,  Benediti,  and  Piccolomini  proving  independently  that 
all  bodies  fall  to  the  earth  with  equal  velocity,  whatever  their  size 
or  weight.  A  century  after,  when  science  had  advanced  to  the 
systematic  application  of  the  higher  mathematics  to  general  phys- 
ics, Newton  and  Leibnitz  discovered  independently  the  differential 
calculus.  A  hundred  years  later  questions  of  molecular  physics 
and  chemistry  were  reached,  and  oxygen  was  discovered  simulta- 
neously by  Priestley  and  Scheele,  and  the  composition  of  water  by 
Cavendish  and  "Watt.  These  discoveries  were  made  because  the 


XVI  INTKODUCTION. 

periods  were  ripe  for  them,  and  we  cannot  doubt  that  if  those  who 
made  them  had  never  lived,  the  labors  of  others  would  have  speed- 
ily attained  the  same  results.  The  discoverer  is,  therefore,  in  a 
great  degree,  but  the  mouthpiece  of  his  time.  Some  discern  clearly 
what  is  dimly  shadowed  forth  to  many;  some  work  out  the  results 
more  completely  than  others,  and  some  seize  the  coming  thought 
so  long  before  it  is  developed  in  the  general  consciousness,  that 
their  announcements  are  unappreciated  and  unheeded.  This  view 
by  no  means  robs  the  discoverer  of  his  honors,  but  it  enables  us  to 
place  upon  them  a  juster  estimate,  and  to  pass  a  more  enlightened 
judgment  upon  the  rival  claims  which  are  constantly  arising  in  the 
history  of  science. 

Probably  the  most  important  event  in  the  general  progress  of 
science  was  the  transition  from  the  speculative  to  the  experimental 
period.  The  ancients  were  prevented  from  creating  science  by  a 
false  intellectual  procedure.  They  believed  they  could  solve  all  the 
problems  of  the  universe  by  thought  alone.  The  moderns  have 
found  that  for  this  purpose  meditation  is  futile  unless  accompanied 
by  observation  and  experiment.  Modern  science,  therefore,  took  its 
rise  in  a  change  of  method,  and  the  adoption  of  the  principle  that 
the  discovery  of  physical  truth  consists  not  in  its  mere  logical  but 
in  its  experimental  establishment.  It  is  now  an  axiom  that  not  he 
who  guesses,  though  he  guess  aright,  is  to  be  adjudged  the  true  dis- 
coverer, but  he  who  demonstrates  the  new  truth,  and  thus  compels 
its  acceptance  into  the  body  of  valid  knowledge. 

Now  the  later  doctrines  of  the  constancy  and  relations  of  forces, 
and  that  heat  is  a  kind  of  motion  among  the  minuter  parts  of  mat- 
ter, have  had  their  twofold  phases  of  history,  corresponding  to  the 
two  methods  of  inquiry.  They  had  an  early  and  vague  recognition 
among  many  philosophers,  and  may  be  traced  in  the  writings  of 
Galileo,  Bacon,  Newton,  Locke,  Leibnitz,  Des  Cartes,  Bernoulli, 
Laplace,  and  others ;  but  they  were  held  by  these  thinkers  as  un- 
verified and  fruitless  speculations,  and  the  subject  awaited  the  gen- 
ius that  could  deal  with  it  according  to  the  more  effective  methods 
of  modern  science. 


SKETCH   OF  THE   CAREER   OF   COUNT  RTJMFORD.        XV11 

It  was  this  country,  widely  reproached  for  being  over-practical, 
which  produced  just  that  kind  of  working  ability  that  was  suited 
to  translate  this  profound  question  from  the  barren  to  the  fruitful 
field  of  inquiry.  It  is  a  matter  of  just  national  pride  that  the  two 
men  who  first  demonstrated  the  capital  propositions  of  pure  sci- 
ence, that  lightning  is  but  a  case  of  common  electricity,  and  that 
heat  is  but  a  mode  of  motion — who  first  converted  these  proposi- 
tions from  conjectures  of  fancy  to  facts  of  science,  were  not  only 
Americans  by  birth  and  education,  but  men  eminently  representa- 
tive of  the  peculiarities  of  American  character — Benjamin  Frank- 
lin and  Benjamin  Thompson,  afterwards  known  as  Count  Eumford. 
The  latter  philosopher  is  less  known  than  the  former,  though  his 
services  to  science  and  society  were  probably  quite  as  great.  The 
prominence  which  his  name  now  occupies  in  connection  with  the 
new  views  of  heat,  and  the  relations  of  forces,  make  it  desirable  to 
glance  briefly  at  his  career. 

BENJAMIN  THOMPSON  was  born  at  "Woburn,  Mass.,  in  1753.  He 
received  the  rudiments  of  a  common  school  education ;  became  a 
merchant's  apprentice  at  twelve,  and  subsequently  taught  school. 
Having  a  strong  taste  for  mechanical  and  chemical  studies,  he  cul- 
tivated them  assiduously  during  his  leisure  time.  At  seventeen  he 
took  charge  of  an  academy  in  the  village  of  Kumford  (now  Con- 
cord), N".  H.,  and  in  1772  married  a  wealthy  widow,  by  whom  he 
had  one  daughter.  At  the  outbreak  of  revolutionary  hostilities  he 
applied  for  a  commission  in  the  American  service,  was  charged 
with  toryism,  left  the  country  in  disgust,  and  went  to  England. 
His  talents  were  there  appreciated,  and  he  took  a  responsible  posi- 
tion under  the  government,  which  he  held  for  some  years. 

After  receiving  the  honor  of  knighthood  he  left  England  and 
entered  the  service  of  the  elector  of  Bavaria.  He  settled  in  Mu- 
nich in  1784,  and  was  appointed  aide-de-camp  and  chamberlain  to 
the  Prince.  The  labors  which  he  now  undertook  were  of  the  most 
extensive  and  laborious  character,  and  could  never  have  been  ac- 


XV111  INTRODUCTION. 

complished  but  for  the  rigorous  habits  of  order  which  he  carried 
into  all  his  pursuits.  He  reorganized  the  entire  military  establish- 
ment of  Bavaria,  introduced  not  only  a  simpler  code  of  tactics,  and 
a  new  system  of  order,  discipline,  and  economy  among  the  troops 
and  industrial  schools  for  the  soldiers'  children,  but  greatly  im- 
proved the  construction  and  modes  of  manufacture  of  arms  and 
ordnance.  He  suppressed  the  system  of  beggary  which  had  grown 
into  a  recognized  profession  in  Bavaria,  and  become  an  enormous 
public  evil — one  of  the  most  remarkable  social  reforms  on  record. 
He  also  devoted  himself  to  various  ameliorations,  such  as  improv- 
ing the  construction  and  arrangement  of  the  dwellings  of  the  work- 
ing classes,  providing  for  them  a  better  education,  organizing  houses 
of  industry,  introducing  superior  breeds  of  horses  and  cattle,  and 
promoting  landscape-gardening,  which  he  did  by  converting  an  old 
abandoned  hunting-ground  near  Munich  into  a  park,  where,  after 
his  departure,  the  inhabitants  erected  a  monument  to  his  honor. 
For  these  services  Sir  Benjamin  Thompson  received  many  distinc- 
tions, and  among  others  was  made  Count  of  the  holy  Eoman  Empire. 
On  receiving  this  dignity  he  chose  a  title  in  remembrance  of  the 
country  of  his  nativity,  and  was  thenceforth  known  as  Count  of 
Rumford. 

His  health  failing  from  excessive  labor  and  what  he  considered 
the  unfavorable  climate,  he  came  back  to  England  in  1798,  and  had 
serious  thoughts  of  returning  to  the  United  States.  Having  re- 
ceived from  the  American  government  the  compliment  of  a  formal 
invitation  to  revisit  his  native  land,  he  wrote  to  an  old  friend  re- 
questing him  to  look  out  for  a  "little  quiet  retreat  "for  himself 
and  daughter  in  the  vicinity  of  Boston.  This  intention,  however, 
failed,  as  he  shortly  after  became  involved  in  the  enterprise  of 
founding  the  Eoyal  Institution  of  England. 

There  was  in  Rumford's  character  a  happy  combination  of  phi- 
lanthropic impulses,  executive  power  in  carrying  out  great  projects, 
and  versatility  of  talent  in  physical  research.  His  scientific  inves- 
tigations were  largely  guided  and  determined  by  his  philanthropic 


SCIENTIFIC   LABORS   OF   COUNT   KUMFOKD.  XIX 

plans  and  public  duties.  His  interest  in  the  more  needy  classes  led 
Mm  to  the  assiduous  study  of  the  physical  wants  of  mankind,  and 
the  best  methods  of  relieving  them ;  the  laws  and  domestic  man- 
agement of  heat  accordingly  engaged  a  large  share  of  his  attention. 
He  determined  the  amount  of  heat  arising  from  the  combustion  of 
different  kinds  of  fuel,  by  means  of  a  calorimeter  of  his  own  in- 
vention. He  reconstructed  the  fireplace,  and  so  improved  the 
methods  of  heating  apartments  and  cooking  food  as  to  produce  a 
saving  in  the  precious  element,  varying  from  one-half  to  seven- 
eighths  of  the  fuel  previously  consumed.  He  improved  the  con- 
struction of  stoves,  cooking  ranges,  coal  grates,  and  chimneys; 
showed  that  the  non-conducting  power  of  cloth  is  due  to  the  air 
enclosed  among  its  fibres,  and  first  pointed  out  that  mode  of  action 
of  heat  called  convection ;  indeed  he  was  the  first  clearly  to  dis- 
criminate between  the  three  modes  of  propagation  of  heat — radia- 
tion, conduction,  and  convection.  He  determined  the  almost  per- 
fect non-conducting  properties  of  liquids,  investigated  the  produc- 
tion of  light,  and  invented  a  mode  of  measuring  it.  He  was  the 
first  to  apply  steam  generally  to  the  warming  of  fluids  and  the 
culinary  art ;  he  experimented  upon  the  use  of  gunpowder,  the 
strength  of  materials,  and  the  maximum  density  of  water,  and 
made  many  valuable  and  original  observations  upon  an  extensive 
range  of  subjects. 

Prof.  James  D.  Forbes,  in  his  able  Dissertation  on  the  recent 
Progress  of  the  Mathematical  and  Physical  Sciences,  in  the  last 
edition  of  the  Encyclopedia  Britannica,  gives  a  full  account  of  Eum- 
ford's  contributions  to  science,  and  remarks : 

"  All  Kumford's  experiments  were  made  with  admirable  precis- 
ion, and  recorded  with  elaborate  fidelity,  and  in  the  plainest  lan- 
guage. Every  thing  with  him  was  reduced  to  weight  and  meas- 
ure, and  no  pains  were  spared  to  attain  the  best  results. 

"Kumford's  name  will  be  ever  connected  with  the  progress  of 
science  in  England  by  two  circumstances :  first,  by  the  foundation 

of  a  perpetual  medal  and  prize  in  the  gift  of  the  council  of  the 
B 


XX  ESTRODTJCTION. 

Eoyal  Society  of  London,  for  the  reward  of  discoveries  connected 
with  heat  and  light;  and  secondly,  by  the  establishment  in  1800  of 
the  Eoyal  Institution  in  London,  destined,  primarily,  for  the  pro- 
motion of  original  discovery,  and,  secondarily,  for  the  diffusion  of  a 
taste  for  science  among  the  educated  classes.  The  plan  was  con- 
ceived with  the  sagacity  which  characterized  Kumford,  and  its  suc- 
cess has  been  greater  than  could  have  been  anticipated.  Davy  was 
there  brought  into  notice  by  Eumford  himself,  and  furnished  with 
the  means  of  prosecuting  his  admirable  experiments.  He  and  Mr. 
Faraday  have  given  to  that  institution  its  just  celebrity  with  little 
intermission  for  half  a  century." 

Leaving  England,  Eumford  took  up  his  residence  in  France,  and 
the  estimation  in  which  he  was  held  may  be  judged  of  by  the  fact 
that  he  was  elected  one  of  the  eight  foreign  associates  of  the  Acad- 
emy of  Sciences. 

Count  Eumford  bequeathed  to  Harvard  University  the  funds 
for  endowing  its  professorship  of  the  Application  of  Science  to  the 
Art  of  Living,  and  instituted  a  prize  to  be  awarded  by  the  Ameri- 
can Academy  of  Sciences,  for  the  most  important  discoveries  and 
improvements  relating  to  heat  and  light.  In  1804  he  married  the 
widow  of  the  celebrated  chemist  Lavoisier,  and  with  her  retired 
to  the  villa  of  Auteuil,  the  residence  of  her  former  husband,  where 
he  died  in  1814. 

Having  thus  glanced  briefly  at  his  career,  I  now  pass  to  the  dis- 
covery upon  which  Count  Eumford's  fame  in  the  future  will  chiefly 
rest.  It  is  described  in  a  paper  published  in  the  transactions  of  the 
Eoyal  Society  for  1798. 

He  was  led  to  it  while  superintending  the  operations  of  the 
Munich  arsenal,  by  observing  Jihe  large  amount  of  heat  generated 
in  boring  brass  cannon.  Eeflecting  upon  this,  he  proposed  to  him- 
self the  following  questions :  "  Whence  comes  the  heat  produced 
in  the  mechanical  operations  above  mentioned? "  "  Is  it  furnished 
by  the  metallic  chips  which  are  separated  from  the  metal  ? " 


XXI 


The  common  hypothesis  affirmed  that  the  heat  produced  had 
been  latent  in  the  metal,  and  had  been  forced  out  by  condensation 
of  the  chips.  But  if  this  were  the  case  the  capacity  for  heat  of  the 
parts  of  metal  so  reduced  to  chips  ought  not  only  to  be  changed, 
but  the  change  undergone  by  them  should  be  sufficiently  great  to 
account  for  all  the  heat  produced.  With  a  fine  saw  Kumford  then 
cut  away  slices  of  the  unheated  metal,  and  found  that  they  had  ex- 
actly the  same  capacity  for  Jieat  as  the  metallic  chips.  No  change 
in  this  respect  had  occurred,  and  it  was  thus  conclusively  proved 
that  the  heat  generated  could  not  have  been  held  latent  in  the 
chips.  Having  settled  this  preliminary  point,  Eumford  proceeds  to 
his  principal  experiments. 

"With  the  intuition  of  the  tru£  investigator,  he  remarks  that 
"  very  interesting  philosophical  experiments  may  often  be  made, 
almost  without  trouble  or  expense,  by  means  of  machinery  con- 
trived for  mere  mechanical  purposes  of  the  arts  and  manufactures." 
Accordingly,  he  mounted  a  metallic  cylinder  weighing  113.13 
pounds  avoirdupois,  in  a  horizontal  position.  At  one  end  there  was 
a  cavity  three  and  a  half  inches  in  diameter,  and  into  this  was  in- 
troduced a  borer,  a  flat  piece  of  hardened  steel,  four  inches  long, 
0.63  inches  thick,  and  nearly  as  wide  as  the  cavity,  the  area  of  con- 
tact of  the  borer  with  the  cylinder  being  two  and  a  half  inches. 
To  measure  the  heat  developed,  a  small  round  hole  was  bored  in 
the  cylinder  near  the  bottom  of  the  cavity,  for  the  insertion  of  a 
small  mercurial  thermometer.  The  borer  was  pressed  against  the 
base  of  the  cavity  with  a  force  of  10,000  pounds,  and  the  cylinder 
made  to  revolve  by  horse-power  at  the  rate  of  thirty-two  times  per 
minute.  At  the  beginning  of  the  experiment  the  temperature  of 
the  air  in  the  shade  and  also  in  the  cylinder  was  60°F.  at  the  end 
of  thirty  minutes,  and  after  the  cylinder  had  made  960  revolutions 
the  temperature  was  found  to  be  130°F. 

Having  taken  away  the  borer,  he  found  that  839  grains  of  me- 
tallic dust  had  been  cut  away.  "  Is  it  possible,"  he%exclaims,  "  that 
the  very  considerable  quantity  of  heat  produced  in  this  experiment 


XX11  ENTEODUCTION. 

— a  quantity  which  actually  raised  the  temperature  of  upward  of 
113  pounds  of  gun  metal  at  least  TO0,  could  have  been  furnished  by 
so  inconsiderable  a  quantity  of  metallic  dust,  and  this  merely  in 
consequence  of  a  change  in  the  capacity  for  heat  ? " 

To  measure  more  precisely  the  heat  produced,  he  next  sur- 
rounded his  cylinder  by  an  oblong  wooden  box  in  such  a  manner 
that  it  could  turn  water-tight  in  the  centre  of  the  box,  while  the 
borer  was  pressed  against  the  bottom.  The  box  was  filled  with 
water  until  the  entire  cylinder  was  covered,  and  the  apparatus  was 
set  in  action.  The  temperature  of  the  water  on  commencing  was 
60°.  He  remarks,  "  The  result  of  this  beautiful  experiment  was 
very  striking,  and  the  pleasure  it  afforded  amply  repaid  me  for  all 
the  trouble  I  had  taken  in  contriving  and  arranging  the  complicated 
machinery  used  in  making  it.  The  cylinder  had  been  in  motion 
but  a  short  time  when  I  perceived,  by  putting  my  hand  into  the 
water  and  touching  the  outside  of  the  cylinder,  that  heat  was  gen- 
erated." 

As  the  work  continued  the  temperature  gradually  rose ;  at  two 
hours  and  twenty  minutes  from  the  beginning  of  the  operation,  the 
water  was  at  200°,  and  in  ten  minutes  more  it  actually  boiled ! 
Upon  this  result  Eumford  observes,  "  It  would  be  difficult  to  de- 
scribe the  surprise  and  astonishment  expressed  in  the  countenances 
of  the  bystanders,  on  seeing  so  large  a  quantity  of  water  heated 
and  actually  made  to  boil  without  any  fire.  Though  there  was 
nothing  that  could  be  considered  very  surprising  in  this  matter, 
yet  I  acknowledge  fairly  that  it  afforded  me  a  degree  of  childish 
pleasure  which,  were  I  ambitious  of  the  reputation  of  a  grave  phi- 
losopher, I  ought  most  certainly  rather  to  hide  than  to  discover." 

Eumford  estimated  the  total  heat  generated  as  sufficient  to  raise 
26.58  pounds  of  ice-cold  water  180°,  or  to  its  boiling  point;  and  he 
adds,  "from  the  results  of  these  computations,  it  appears  that  the 
quantity  of  heat  produced  equally  or  in  a  continuous  stream,  if  I 
may  use  the  expression,  by  the  friction  of  the  blunt  steel  borer 
against  the  bottom  of  the  hollow  metallic  cylinder,  was  greater 


XX111 


than  that  produced  in  the  combustion  of  nine  wax  candles,  each 
three-quarters  of  an  inch  in  diameter,  all  burning  together  with 
clear  bright  flames." 

"  One  horse  would  have  been  equal  to  the  work  performed, 
though  two  were  actually  employed.  Heat  may  thus  be  produced 
merely  by  the  strength  of  a  horse,  and  in  a  case  of  necessity  this 
might  be  used  in  cooking  victuals.  But  no  circumstances  could  be 
imagined  in  which  this  method  of  producing  heat  could  be  advan- 
tageous, for  more  heat  might  be  obtained  by  using  the  fodder  ne- 
cessary for  the  support  of  the  horse,  as  fuel. 

"  By  meditating  on  the  results  of  all  these  experiments,  we  are 
naturally  brought  to  that  great  question  which  has  so  often  been 
the  subject  of  speculation  among  philosophers,  namely,  What  is 
heat?  Is  there  such  a  thing  as  an  igneous  fluid?  Is  there  any 
thing  that  with  propriety  can  be  called  caloric  ? 

"  We  have  seen  that  a  very  considerable  quantity  of  heat  may 
be  excited  by  the  friction  of  two  metallic  surfaces,  and  given  off  in 
a  constant  stream  or  flux  in  all  directions,  without  interruption  or 
intermission,  and  without  any  signs  of  diminution  or  exhaustion. 
In  reasoning  on  this  subject  we  must  not  forget  that  most  remark- 
able circumstance,  that  the  source  of  the  heat  generated  by  friction 
in  these  experiments  appeared  evidently  to  be  inexhaustible.  (The 
italics  are  Eumford's.)  It  is  hardly  necessary  to  add,  that  any 
thing  which  any  insulated,  body  or  system  of  bodies  can  continue 
to  furnish  without  limitation,  cannot  possibly  be  a  material  sub- 
stance ;  and  it  appears  to  me  to  be  extremely  difficult,  if  not  quite 
impossible,  to  form  any  distinct  idea  of  any  thing  capable  of  being 
excited  and  communicated  in  those  experiments,  except  it  be  MO- 
TION." 

No  one  can  read  the  remarkably  able  and  lucid  paper  from 
which  these  extracts  are  taken,  without  being  struck  with  the  per- 
fect distinctness  with  which  the  problem  to  be  solved  was  pre- 
sented, and  the  systematic  and  conclusive  method  of  its  treatment. 
Bumford  kept  strictly  within  the  limits  of  legitimate  inquiry,  which 


XXIV  INTEODtJCTION. 

no  man  can  define  better  than  he  did.  "  I  am  very  far  from  pre- 
tending to  know  how,  or  by  what  means  or  mechanical  contri- 
vances, that  particular  kind  of  motion  in  bodies,  which  has  been 
supposed  to  constitute  heat,  is  exerted,  continued,  and  propagated, 
and  I  shall  not  presume  to  trouble  the  Society  with  new  conjec- 
tures. But  although  the  mechanism  of  heat  should  in  part  be  one 
one  of  those  mysteries  of  nature,  which  are  beyond  the  reach  of 
human  intelligence,  this  ought  by  no  means  to  discourage  us,  or 
even  lessen  our  ardor  in  our  attempts  to  investigate  the  laws  of  its 
operations.  How  far  can  we  advance  in  any  of  the  paths  which 
science  has  opened  to  us,  before  we  find  ourselves  enveloped  in 
those  thick  mists,  which  on  every  side  bound  the  horizon  of  the 
human  intellect." 

Rumford's  experiments  completely  annihilated  the  material  hy- 
pothesis of  heat,  while  the  modern  doctrine  was  stated  in  explicit 
terms.  He  moreover  advanced  the  question  to  its  quantitative  and 
highest  stage,  proposing  to  find  the  numerical  relation  between 
mechanical  power  and  heat,  and  obtained  a  result  remarkably  near 
to  that  finally  established.  The  English  unit  of  force  is  the  foot- 
pound, that  is,  one  pound  falling  through  one  foot  of  space ;  the 
unit  of  heat  is  one  pound  of  water  heated  1°  F.  Just  fifty  years 
subsequently  to  the  experiment  of  Rumford,  Dr.  J.  P.  Joule,*  of 
Manchester,  England,  after  a  most  delicate  and  elaborate  series  of 
experiments,  determined  that  772  units  of  force  produce  one  unit 
of  heat ;  that  is,  772  pounds  falling  through  one  foot  produces  suf- 
ficient heat  to  raise  one  pound  of  water  1°  F.  This  law  is  known 
as  the  mechanical  equivalent  of  heat.  Now,  when  we  throw  Rum- 
ford's  results  into  these  terms,  we  find  that  about  940  units  of  force 
produced  a  unit  of  heat,  and  that,  therefore,  on  a  large  scale,  and 
at  the  very  first  trial,  he  came  within  twenty  per  cent,  of  the  true 

*  JAMES  PBESCOTT  JOULE,  born  December  24th,  1818,  at  Salford,  near  Manchester, 
England,  where  he  pursued  the  occupation  of  a  brewer.  Long  and  deeply  devoted 
to  scientific  investigation,  he  became  a  member  of  the  Manchester  Philosophical  So- 
ciety in  1842,  and  of  the  Eoyal  Society  of  London  in  1850. 


SUMMARY  OF  RUMFOKD's   CLAIMS.  XXV 

statement.  ISTo  account  was  taken  of  the  heat  lost  by  radiation, 
which,  considering  the  high  temperature  produced,  and  the  dura- 
tion of  the  experiment,  must  have  been  considerable ;  so  that  as 
Rumford  himself  noticed,  this  value  must  be  too  high.  The  ear- 
liest numerical  results  in  science  are  rarely  more  than  rough  ap- 
proximations, yet  they  may  guide  to  the  establishment  of  great 
principles.  Certainly  no  one  could  question  Dalton's  claim  to  the 
discovery  of  the  law  of  definite  proportions,  because  of  the  inac- 
curacy of  the  numbers  upon  which  he  first  rested  it. 

We  are  called  further  to  note  that  Rumford's  ideas  upon  the 
general  subject  of  forces  were  far  in  advance  of  his  age.  He  saw 
the  relation  of  all  friction  to  heat,  and  suggested  that  of  fluids,  by 
churning  processes,  as  a  means  of  producing  it — precisely  the 
method  finally  employed  by  Joule  in  establishing  the  mechanical 
equivalent  of  heat.  He  furthermore  regarded  animals  dynami- 
cally, considering  their  force  as  the  derivative  of  their  food,  and 
therefore  as  not  created.  That  Rumford  held  these  views  in  the 
comprehensive  and  matured  sense  in  which  they  are  now  enter- 
tained is,  of  course,  not  asserted.  The  advance  from  his  day  to 
ours  has  been  prodigious.  Whole  sciences  have  been  created, 
which  afford  the  most  beautiful  exemplifications  of  the  new  doc- 
trines. Those  doctrines  have  received  their  subsequent  develop- 
ment in  various  directions  by  many  minds,  but  we  may  be  allowed 
to  question  if  the  contributions  of  any  of  their  promoters  will  sur- 
pass, if  indeed  they  will  equal,  the  value  and  importance  which  we 
must  assign  to  the  first  great  experimental  step  in  the  new  direc- 
tion. 

The  claims  of  Rumford  may  be  summarized  as  follows : 

I.  He  was  the  man  who  first  took  the  question  of  the  nature 
of  heat  out  of  the  domain  of  metaphysics,  where  it  had 
been  speculated  upon  since  the  time  of  Aristotle,  and 
placed  it  upon  the  true  basis  of  physical  experiment. 

II.  He  first  proved  the  insufficiency  of  the  current  explanations 


XXVI  INTRODUCTION. 

of  the  sources  of  heat,  and  demonstrated  the  falsity  of  the 
prevailing  view  of  its  materiality. 

III.  He  first  estimated  the  quantitative  relation  between  the  heat 

produced  by  friction  and  that  by  combustion. 

IV.  He  first  showed  the  quantity  of  heat  produced  by  a  definite 

amount  of  mechanical  work,  and  arrived  at  a  result  re- 
markably near  the  finally  established  law. 

• 

V.  He  pointed  out  other  methods  to  be  employed  in  determining 
the  amount  of  heat  produced  by  the  expenditure  of  me- 
chanical power,  instancing  particularly  the  agitation  of 
water,  or  other  liquids,  as  in  churning. 

VI.  He  regarded  the  power  of  animals  as  due  to  their  food,  there- 
fore as  having  a  definite  source  and  not  created,  and  thus 
applied  his  views  of  force  to  the  organic  world. 

VH.  Rumford  was  the  first  to  demonstrate  the  quantitative  con- 
vertibility of  force  in  an  important  case,  and  the  first  to 
reach,  experimentally,  the  fundamental  conclusion  that  heat 
is  but  a  mode  of  motion. 

In  his  late  work  upon  heat,  Prof.  Tyndall,  after  quoting  co- 
piously from  Eumford's  paper,  remarks:  "When  the  history  of  the 
dynamical  theory  of  heat  is  written,  the  man  who  in  opposition  to 
the  scientific  belief  of  his  time  could  experiment,  and  reason  upon 
experiment,  as  did  Kumford  in  the  investigation  here  referred  to, 
cannot  be  lightly  passed  over."  Had  other  English  writers  been 
equally  just,  there  would  have  been  less  necessity  for  the  foregoing 
exposition  of  Eumford's  labors  and  claims ;  but  there  has  been  a 
manifest  disposition  in  various  quarters  to  obscure  and  depreciate 
them.  Dr.  Whewell,  in  his  history  of  the  Inductive  Sciences, 
treats  the  subject  of  thermotics  without  mentioning  him.  An  em- 
inent Edinburgh  professor,  writing  recently  in  the  Philosophical 
Magazine,  under  the  confessed  influence  of  'patriotism,'  under- 


^   RELATION   TO  THE   QUESTION.  XXVU 


takes  to  make  the  dynamical  theory  of  heat  an  English  monopoly, 
due  to  Sir  Isaac  Newton,  Sir  Humphrey  Davy,  and  Dr.  J.  P. 
Joule  ;  while  an  able  writer  in  a  late  number  of  the  North  British 
Review,  in  sketching  the  historic  progress  of  the  new  views,  puts 
Davy  forward  as  their  founder,  and  assigns  to  Rumford  a  minor 
and  subsequent  place. 

Sir  Humphrey  Davy,  it  is  well  known,  early  rejected  the  caloric 
hypothesis.  In  1799,  at  the  age  of  twenty-one,  he  published  a 
tract  at  Bristol,  describing  some  ingenious  experiments  upon  the 
subject.  It  was  the  publication  of  this  pamphlet  which  brought 
him  to  Eumford's  notice,  and  resulted  in  his  subsequent  connection 
with  the  Royal  Institution.  But  Davy's  ideas  upon  the  question 
were  far  from  clear,  and  will  bear  no  comparison  with  those  of 
Rumford,  published  the  year  before.  Indeed  his  eulogist  remarks  : 
"It  is  certain  that  even  Davy  himself  was  led  astray  in  his  argu- 
ment by  using  the  hypothesis  of  change  of  capacity  as  the  basis 
of  his  reasoning,  and  that  he  might  have  been  met  successfully  by 
any  able  calorist,  who,  though  maintaining  the  materiality  of  heat, 
might  have  been  willing  to  throw  overboard  one  or  two  of  the  less 
essential  tenets  of  his  school  of  philosophy."  It  was  not  till  1812 
that  Davy  wrote  in  his  Chemical  Philosophy,  "  The  immediate 
cause  of  the  phenomena  of  heat  then  is  motion,  and  the  laws  of  its 
communication  are  precisely  the  same  as  those  of  the  communica- 
tion of  motion."  "When,  therefore,  we  remember  that  Davy's  first 
publication  was  subsequent  to  that  of  Rumford's,  that  he  confined 
himself  to  the  narrowest  point  of  the  subject,  the  simple  question 
of  the  existence  of  caloric  ;  and  that  he  nowhere  gives  evidence 
of  having  the  slightest  notion  of  the  quantitative  relation  between 
mechanical  force  and  heat,  the  futility  of  the  claim  which  would 
make  him  the  experimental  founder  of  the  dynamical  theory,  is 
abundantly  apparent. 

The  inquiries  opened  by  Rumford  and  Davy  were  not  formally 
pursued  by  the  succeeding  generation.  Even  the  powerful  adhe- 
sion of  Dr.  Thomas  Young  —  perhaps  the  greatest  mind  in  science 


XXV1U  INTRODUCTION. 

since  Newton—- failed  to  give  currency  to  the  new  views.  But  the 
salient  and  impregnable  demonstration  of  Rumford,  and  the  ingen- 
ious experiments  of  Davy,  facts  which  could  neither  be  evaded  nor 
harmonized  with  the  prevailing  errors,  were  not  without  influence. 
That  there  was  a  general,  though  unconscious  tendency  toward  a 
new  philosophy  of  forces,  in  the  early  inquiries  of  the  present  cen- 
;  tury,  is  shown  by  the  fact  that  various  scientific  men  of  different 
nations,  and  with  no  knowledge  of  each  other's  labors,  gave  ex- 
pression to  the  same  views  at  about  the  same  time.  Grove  and 
Joule  of  England,  Mayer  of  Germany,  and  Oolding  of  Denmark, 
announced  the  general  doctrine  of  the  mutual  relations  of  the  forces, 
with  more  or  less  explication,  about  1842,  and  Seguin  of  France, 
it  is  claimed,  a  little  earlier.  From  this  time  the  subject  was  closely 
pursued,  and  the  names  of  Helmholtz,  Holtzman,  Clausius,*  Faraday, 
Thompson,  Eankine,t  Tyndall,  Carpenter,  and  others  are  intimately 
associated  with  its  advancement.  In  this  country  Professors  Henry  J 
and  Leconte  §  have  contributed  to  illustrate  the  organic  phase  of  the 
doctrine. 

I  cannot  here  attempt  an  estimate  of  the  respective  shares 
which  these  men  have  had  in  constructing  the  new  theories ;  the 
reader  will  gather  various  intimations  upon  this  point  from  the 
succeeding  essays.  The  foreign  periodicals,  both  scientific  and  lit- 
erary, show  that  the  question  is  being  thoroughly  sifted,  and  mate- 
rials accumulating  for  the  future  history  of  the  subject.  The  para- 
mount claims  are,  however,  those  of  Joule,  Mayer,  and  Grove. 

*  CLATTSITTS,  EtnooLPH  JULIUS  IMMANTTEL  was  born  at  Coslin,  Pommern,  January 
22, 1822.  He  became  Professor  of  Philosophy  and  Physics  in  the  Polytechnic  School 
at  Zurich  in  1865,  and  then  Professor  of  the  Zurich  University  (1857).  He  was  after- 
wards teacher  of  Physics  and  Artillery  in  the  School  of  Berlin,  and  then  private 
teacher  of  the  University  of  that  place. 

t  KANKINE,  WILLIAM  JOHN  MACQTTOBN  was  born  at  Edinburgh,  July  5, 1820.  He 
is  a  civil  engineer  in  Glasgow,  a  member  of  the  Philosophical  Society  at  that  place, 
and  of  the  Eoyal  Society  of  London. 

$  See  the  article  "Meteorology,"  in  the  Agricultural  Report  of  the  Patent  Office,  for 
1857. 

§  See  the  American  Journal  of  Science  for  Nov.  1859. 


CLAIMS   OF  JOULE,  GKOVE,  AND  MAYER.  XXIX 

According  to  the  strict  rule  of  science,  that  in  all  those  cases 
where  experimental  proof  is  possible,  he  who  first  supplies  it  is  the 
true  discoverer,  Dr.  Joule  must  he  assigned  the  foremost  place 
among  the  modern  investigators  of  the  subject.  He  dealt  with  the 
whole  question  upon  the  basis  of  experiment.  He  labored  with 
great  perseverance  and  skill  to  determine  the  mechanical  equivalent 
of  heat — the  corner-stone  of  the  edifice ;  and  in  accomplishing  this 
result  in  1850,  he  may  be  said  to  have  matured  the  work  of  Kumford, 
and  finally  established  upon  an  experimental  basis  the  great  law  of 
thermo-dynamics,  to  remain  a  demonstration  of  science  forever. 

Professor  Grove  has  also  worked  out  the  subject  in  his  own  in- 
dependent way.  Combining  original  experimental  investigations 
of  great  acuteness,  with  the  philosophic  employment  of  the  gen- 
eral results  of  science,  he  was  the  first  to  give  complete  and  system- 
atic expression  to  the  new  views.  His  able  work,  which  opens 
the  present  series,  is  an  authoritative  exposition,  and  an  acknowl- 
edged classic  upon  the  subject. 

Again,  the  claims  of  Dr.  Mayer  to  an  eminent  and  enviable 
place  among  the  pioneers  of  this  great  scientific  movement,  are  un- 
questionable. There  has  evidently  been,  on  the  part  of  some  Eng- 
glish  writers,  an  unworthy  inclination  to  depreciate  his  merits, 
which  has  given  rise  to  a  sharp  and  searching  controversy.  The 
intellectual  rights  of  the  German  philosopher  have,  however,  been 
decisively  vindicated  by  the  chivalric  pen  of  Prof.  Tyndall ;  and  it 
is  to  the  public  interest  thus  excited,  that  we  are  indebted  for  the 
translation  of  Mayer's  papers,  which  appear  in  this  volume.  Mayer 
did  not  experiment  to  the  extent  of  Joule  and  Grove,  yet  he  well 
knew  its  importance,  and  made  such  investigations  as  his  apparatus 
and  the  duties  of  a  laborious  profession  would  allow.  Yet  his 
views  were  not  therefore  mere  ingenious  and  probable  conjectures. 
Master  of  the  results  of  modern  science,  and  of  the  mathematical 
methods  of  dealing  with  them,  possessing  a  broad  philosophic 
grasp,  and  an  extraordinary  mental  pertinacity,  Dr.  Mayer  entered 
early  upon  the  inquiry,  and  not  only  has  he  developed  many  of  its 


XXX  INTEODUCTION. 

prime  applications  in  advance  of  any  other  thinker,  but  he  has 
done  his  work  under  circumstances  and  in  a  manner  which  awa- 
kens the  highest  admiration  for  his  genius.* 

An  eminent  authority  has  remarked  *  that  these  discoveries  open 
a  region  which  promises  possessions  richer  than  any  hitherto 
granted  to  the  intellect  of  man.'  Involving  as  they  do  a  revolution 
of  fundamental  ideas,  their  consequences  must  be  as  comprehen- 
sive as  the  range  of  human  thought.  A  principle  has  been  devel- 
oped of  all-pervading  application,  which  brings  the  diverse  and 
distant  branches  of  knowledge  into  more  intimate  and  harmonious 
alliance,  and  affords  a  profounder  insight  into  the  universal  order. 
Not  only  is  science  itself  deeply  affected  by  the  presentation  of  its 
questions,  in  new  and  suggestive  lights,  but  its  method  is  at  once 
made  universal.  There  is  a  crude  notion  in  many  minds,  that  it  j8 
the  business  of  science  to  occupy  itself  merely  with  the  study  of 
matter.  "When,  hitherto,  it  has  pressed  its  inquiries  into  the  higher 

*  Prof.  Tyndall  remarks :  "  Mayer  probably  had  not  the  means  of  making  experi- 
ments himself,  but  he  ransacked  the  records  of  experimental  science  for  his  data,  and 
5^    *>„        thus  conferred  upon  his  writings  a  strength  which  mere  speculation  can  never  possess^. 
From  the  extracts  which  I  have  given,  the  reader  may  infer  his  strong  desire  for  quan- 
t      V       titative  accuracy,  the  clearness  of  his  insight,  and  the  firmness  of  his  grasp.    Kegard- 
^  \J^  ing  the  recognition  which  will  be  ultimately  accorded  to  Dr.  Mayer,  a  shade  of  trouble 
doubt  has  never  crossed  my  mind.    Individuals  may  seek  to  pull  him  down,  but 
-^  "their  efforts  will  be  unavailing  as  long  as  such  evidence  of  his  genius  exists,  and  as 
long  as  the  general  mind  of  humanity  is  influenced  by  considerations  of  justice  and 
truth. 

vy  "  The  paucity  of  facts  in  Mayer's  time  has  been  urged  as  if  it  were  a  reproach  to 

him ;  but  it  ought  to  be  remembered  that  the  quantity  of  fact  necessary  to  a  generaliza- 
'  ^__,  tion  is  different  for  different  minds.  'A  word  to  the  wise  is  sufficient  for  them,1  and 
a  single  fact  in  some  minds  bears  fruit  that  a  hundred  cannot  produce,  in  others. 
.Mayer's  data  were  comparatively  scanty,  but  his  genius  went  far  to  supply  the  lack  of 
experiment,  by  enabling  him  to  see  clearly  the  bearing  of  such  facts  as  he  possessed. 
They  enabled  him  to  think  out  the  law  of  conservation,  and  his  conclusions  received 
the  stamp  of  certainty  from  the  subsequent  experimental  labors  of  Mr.  Joule.  In  ref- 
erence to  their  comparative  merits,  I  would  say  that  as  Seer  and  Generalizer,  Mayer, 
in  my  opinion,  stands  first-^ts  experimental  philosopher,  Joule.'1'' 


THE   TRUE   SCOPE   OF   SCIENCE.  XXXI 

region  of  life,  mind,  society,  history,  and  education,  the  traditional 
custodians  of  these  subjects  have  bidden  it  keep  within  its  limits 
and  stick  to  matter.  But  science  is  not  to  be  hampered  by  this 
narrow  conception ;  its  office  is  nothing  less  than  to  investigate  the 
laws  and  universal  relations  of  force,  and  its  domain  is  therefore 
coextensive  with  the  display  of  power.  Indeed,  as  we  know  noth- 
ing of  matter,  except  through  its  manifestation  of  forces,  it  is  ob- 
vious that  the  study  of  matter  itself  is  at  last  resolved  into  the 
study  of  forces.  The  establishment  of  a  new  philosophy  of  forces, 
therefore,  by  its  vast  extension  of  the  scope  and  methods  of  sci- 
ence, constitutes  a  momentous  event  of  intellectual  progress. 

The  discussions  of  the  present  volume  will  make  fully  apparent 
the  importance  of  the  new  doctrines  in  relation  to  physical  science, 
but  their  higher  implications  are  but  partially  unfolded.  In  the 
concluding  article  Dr.  Carpenter  has  shown  the  applicability  of  the 
principle  of  correlation  to  vital  phenomena.  His  argument  is  of 
interest,  not  only  because  of  the  facts  and  principles  established, 
but  as  opening  an  inquiry  which  must  lead  to  still  larger  results : 
for,  if  the  principle  be  found  operative  in  fundamental  organic 
processes,  it  will  undoubtedly  be  traced  in  those  which  are  higher ; 
if  in  the  lower  sphere  of  life,  then  throughout  that  sphere.  If  the 
forces  are  correlated  in  organic  growth  and  nutrition,  they  must  bo 
in  organic  action ;  and  thus  human  activity,  in  all  its  forms,  is 
brought  within  the  operation  of  the  law.  As  a  creature  of  or- 
ganic nutrition,  borrowing  matter  and  force  from  the  outward 
world ;  as  a  being  of  feeling  and  sensibility,  of  intellectual  power 
and  multiform  activities,  man  must  be  regarded  as  amenable  to  the 
great  law  that  forces  are  convertible  and  indestructible;  and  as 
psychology  and  sociology — the  science  of  mind  and  the  science  of 
society — have  to  deal  constantly  with  different  phases  and  forms 
of  human  energy,  the  new  principle  must  be  of  the  profoundest 
import  in  relation  to  these  great  subjects. 

The  forces  manifested  in  the  living  system  are  of  the  most 
varied  and  unlike  character,  mechanical,  thermal,  luminous,  electric, 


XXX11  INTRODUCTION. 

chemical,  nervous,  sensory,  emotional,  and  intellectual.  That  these 
forces  are  perfectly  coordinated — that  there  is  some  definite  relation 
among  them  which  explains  the  marvellous  dynamic  unity  of  the 
living  organism,  does  not  admit  of  question.  That  this  relation  is 
of  the  same  nature  as  that  which  is  found  to  exist  among  the 
purely  physical  forces,  and  which  is  expressed  by  the  term  '  Correl- 
ation,' seems  also  abundantly  evident.  From  the  great  complex- 
ity of  the  conditions,  the  same  exactness  will  not,  of  course,  be 
expected  here  as  in  the  inorganic  field,  but  this  is  one  of  the  neces- 
sary limitations  of  all  physiological  and  psychological  inquiry ;  thus 
qualified  the  proofs  of  the  correlation  of  the  nervous  and  mental 
forces  with  the  physical,  are  as  clear  and  decisive  as  those  for  the 
physical  forces  alone. 

If  a  current  of  electricity  is  passed  through  a  small  wire  it 
produces  heat,  while  if  heat  is  applied  to  a  certain  combination  of 
metals,  it  reproduces  a  current  of  electricity;  these  forces  are, 
therefore,  correlated.  A  current  of  electricity  passed  through  a 
small  portion  of  a  motor  or  sensory  nerve  will  excite  the  nerve- 
force  in  the  remainder,  while,  on  the  other  hand,  as  is  shown  in  the 
case  of  the  torpedo,  the  nerve-force  may  generate  electricity. 
Nerve-force  may  produce  heat,  light,  electricity,  and,  as  we  con- 
stantly experience,  mechanical  power,  and  these  in  their  turn  may 
also  excite  nerve-force.  This  form  of  energy  is  therefore  clearly 
entitled  to  a  place  in  the  order  of  correlated  agencies. 

Again,  if  we  take  the  highest  form  of  mental  action,  viz. :  will- 
power, we  find  that  while  it  commands  the  movements  of  the  sys- 
tem, it  does  not  act  directly  upon  the  muscles,  but  upon  the  cerebral 
hemispheres  of  the  brain.  There  is  a  dynamic  chain  of  which 
voluntary  power  is  but  one  link.  The  will  is  a  power  which  excites 
nerve-force  in  the  brain,  which  again  excites  mechanical  power  in 
the  muscles.  Will-power  is  therefore  correlated  with  nerve-power 
in  the  same  manner  as  the  latter  is  with  muscular  power.  Dr. 
Carpenter  well  observes:  "It  is  difficult  to  see  that  the  dynamical 
agency  which  we  term  will  is  more  removed  from  nerve-force  on 


CORRELATION  OF  NERVOUS  AND  MENTAL  FORCES.   XXX1U 

the  one  hand  than  nerve-force  is  removed  from  motor  force  on  the 
other*  Each,  in  giving  origin  to  the  next,  is  itself  expended  or 
ceases  to  exist  as  swch,  and  each  bears,  in  its  own  intensity,  a  pre- 
cise relation  to  that  of  its  antecedent  and  its  consequent."  We  have 
here  only  space  briefly  to  trace  the  principle  in  its  application  to 
sensations,  motions,  and  intellectual  operations. 

The  physical  agencies  acting  upon  inanimate  objects  in  the 
external  world,  change  their  form  and  state,  and  we  regard  these 
changes  as  transformed  manifestations  of  the  forces  in  action.  A 
body  is  heated  by  hammering ;  the  heat  is  but  transmuted  mechani- 
cal force ;  or  a  body  is  put  in  motion  by  heat,  a  certain  quantity 
being  transformed  into  mechanical  effect,  or  motion  of  the  mass. 
And  so  it  is  held  that  no  force  can  arise  except  by  the  expenditure 
of  a  preexisting  force.  Now,  the  living  system  is  acted  upon  by 
the  same  agencies  and  under  the  same  law.  Impressions  made 
upon  the  organs  of  sense  give  rise  to  sensations,  and  we  have  the 
same  warrant  in  this,  as  in  the  former  case,  for  regarding  the  effects 
as  transformations  of  the  forces  in  action.  If  the  change  of 
molecular  state  in  a  melted  body  represents  the  heat  transformed 
in  fusing  it,  so  the  sensation  of  warmth  in  a  living  body  must 
represent  the  heat  transformed  in  producing  it.  The  impression  on 
the  retina,  as  well  as  that  on  the  photographic  tablet,  results  from 
the  transmuted  impulses  of  light.  And  thus  impressions  made 
from  moment  to  moment  on  all  our  organs  of  sense,  are  directly 
correlated  with  external  physical  forces.  This  correlation,  further- 
more, is  quantitative  as  well  as  qualitative.  Not  only  does  the 
light-force  produce  its  peculiar  sensations,  but  the  intensity  of  these 
sensations  corresponds  with  the  intensity  of  the  force ;  not  only  is 
atmospheric  vibration  transmuted  into  the  sense  of  sound,  but  the 
energy  of  the  vibration  determines  its  loudness.  And  so  in  all 
other  cases ;  the  quantity  of  sensation  depends  upon  the  quantity 
of  the  force  acting  to  produce  it. 

Moreover,  sensations  do  not  terminate  in  themselves,  or  come 
to  nothing ;  they  produce  certain  correlated  and  equivalent  effects. 


XXXIV  INTRODUCTION. 

The  feelings  of  light,  heat,  sound,  odor,  taste,  pressure,  are  im- 
mediately followed  by  physiological  effects,  as  secretion,  muscular 
action,  &c.  Sensations  increase  the  contractions  of  the  heart,  and 
it  has  been  lately  maintained  that  every  sensation  contracts  the 
muscular  fibres  throughout  the  whole  vascular  system.  The  res- 
piratory muscles  also  respond  to  sensations ;  the  rate  of  breathing 
being  increased  by  both  pleasurable  and  painful  nerve-impressions. 
The  quantity  of  sensation,  moreover,  controls  the  quantity  of  emo- 
tion. Loud  sounds  produce  violent  starts,  disagreeable  tastes  cause 
wry  faces,  and  sharp  pains  give  rise  to  violent  struggles.  Even 
when  groans  and  cries  are  suppressed,  the  clenched  hands  and  set 
teeth  show  that  the  muscular  excitement  is  only  taking  another 
direction. 

Between  the  emotions  and  bodily  actions  the  correlation  and 
equivalence  are  also  equally  clear.  Moderate  actions,  like  moderate 
sensations,  excite  the  heart,  the  vascular  system,  and  the  glandular 
organs.  As  the  emotions  rise  in  strength,  however,  the  various 
systems  of  muscles  are  thrown  into  action ;  and  when  they  reach  a 
certain  pitch  of  intensity,  violent  convulsive  movements  ensue. 
Anger  frowns  and  stamps ;  grief  wrings  its  hands ;  joy  dances  and 
leaps — the  amount  of  sensation  determining  the  quantity  of  correla- 
tive movement. 

Dr.  Carpenter,  in  his  Physiology,  has  brought  forward  numerous 
exemplifications  of  this  principle  of  the  conversion  of  emotion  into 
movement,  as  seen  in  the  common  workings  of  human  nature. 
Most  persons  have  experienced  the  difficulty  of  sitting  still  under 
high  excitement  of  the  feelings,  and  also  the  relief  afforded  by 
walking  or  active  exercise ;  while,  on  the  other  hand,  repression  of 
the  movements  protracts  the  emotional  excitement.  Many  irascible 
persons  get  relief  from  their  irritated  feelings  by  a  hearty  explosion 
of  oaths,  others  by  a  violent  slamming  of  the  door,  or  a  prolonged 
fit  of  grumbling.  Demonstrative  persons  habitually  expend  their 
feelings  in  action,  while  those  who  manifest  them  less  retain  them 
longer:  hence  the  former  are  more  weak  and  transient  in  their 


CORRELATION  OF  PHYYSICAL  AND  MENTAL  FORCES.    XXXV 

attachments  than  the  latter,  whose  unexpended  emotions  become 
permanent  elements  of  character.  For  the  same  reason,  those  who 
are  loud  and  vehement  in  their  lamentations  seldom  die  of  grief; 
while  the  deep-seated  emotions  of  sorrow  which  others  cannot 
work  off  in  violent  demonstrations,  depress  the  organic  functions, 
and  often  wear  out  the  life. 

The  intellectual  operations  are  also  directly  correlated  with 
physical  activities.  As  in  the  inorganic  world  we  know  nothing 
of  forces  except  as  exhibited  by  matter,  so  in  the  higher  intellectual 
realm  we  know  nothing  of  mind-force  except  through  its  material 
manifestations.  Mental  operations  are  dependent  upon  material 
changes  in  the  nervous  system ;  and  it  may  now  be  regarded  as  a 
fundamental  physiological  principle,  that  "  no  idea  or  feeling  can 
arise,  save  as  the  result  of  some  physical  force  expended  in  pro- 
ducing it."  The  directness  of  this  dependence  is  proved  by  the 
fact  that  any  disturbance  of  the  train  of  cerebral  transformations 
disturbs  mentality,  while  their  arrest  destroys  it.  And  here,  also, 
the  correlation  is  quantitative.  Other  things  being  equal  there  is  a 
relation  between  the  size  of  the  nerve  apparatus  and  the  amount 
of  mental  action  of  which  it  is  capable.  Again,  it  is  dependent 
upon  the  vigor  of  the  circulation ;  if  this  is  arrested  by  the  cessation 
of  the  heart's  action,  total  unconsciousness  results ;  if  it  is  enfeebled, 
mental  action  is  low ;  while  if  it  is  quickened,  mentality  rises,  even 
to  delirium,  when  the  cerebral  activity  becomes  excessive.  Again, 
the  rate  of  brain  activity  is  dependent  upon  the  special  chemical 
ingredients  of  the  blood,  oxygen  and  carbon.  Increase  of  oxygen 
augments  cerebral  action,  while  increase  of  carbonic  acid  depresses 
it.  The  degree  of  mentality  is  also  dependent  upon  the  phosphatic 
constituents  of  the  nervous  system.  The  proportion  of  phosphorus 
in  the  brain  is  smallest  in  infancy,  idiocy,  and  old  age,  and  greatest 
during  the  prime  of  life ;  while  the  quantity  of  alkaline  phosphates 
excreted  by  the  kidneys  rises  and  falls  with  the  variations  of  mental 
activity.  The  equivalence  of  physical  agencies  and  mental  effects 
is  still  further  seen  in  the  action  of  various  substances,  as  alcohol, 


w 
*/ 


)/ 


XXXVI  INTRODUCTION. 

opium,  hashish,  nitrous  oxide,  etc.,  when  absorbed  into  the  blood. 
Within  the  limits  of  their  peculiar  action  upon  the  nervous  centres, 
the  effect  of  each  is  strictly  proportionate  to  the  quantity  taken. 
There  is  a  constant  ratio  between  the  antecedents  and  consequents. 
"  How  this  metamorphosis  takes  place — how  a  force  existing 
as  motion,  heat,  or  light,  can  become  a  mode  of  consciousness — 
*5  how  it  is  possible  for  aerial  vibrations  to  generate  the  sensation 
we  call  sound,  or  for  the  forces  liberated  by  chemical  changes  in  the 
^  \j  brain,  to  give  rise  to  emotion,  these  are  mysteries  which  it  is  im- 
possible to  fathom.  But  they  are  not  profounder  mysteries  than 
the  transformation  of  the  physical  forces  into  each  other.  They 
are  not  more  completely  beyond  our  comprehension  than  the 
natures  of  mind  and  matter.  They  have  simply  the  same  insolu- 
bility as  all  other  ultimate  questions.  "We  can  learn  nothing  more 
than  that  here  is  one  of  the  uniformities  in  the  order  of  phe- 
nomena." 

The  law  of  correlation  being  thus  applicable  to  human  energy 
as  well  as  to  the  powers  of  nature,  it  must  also  apply  to  society, 
where  we  constantly  witness  the  conversion  of  forces  on  a  compre- 
hensive scale.  The  powers  of  nature  are  transformed  into  the  activ- 
ities of  society;  water-power,  wind-power,  steam-power,  and  electri- 
cal-power are  pressed  into  the  social  service,  reducing  human  labor, 
multiplying  resources,  and  carrying  on  numberless  industrial  pro- 
cesses :  indeed,  the  conversion  of  these  forces  into  social  activities 
is  one  of  the  chief  triumphs  of  civilization.  The  universal  forces 
of  heat  and  light  are  transformed  by  the  vegetable  kingdom  into 
the  vital  energy  of  organic  compounds,  and  then,  as  food,  are  again 
converted  into  human  beings  and  human  power.  The  very  exist- 
ence as  well  as  the  activity  of  society  are  obviously  dependent  upon 
the  operations  of  vegetable  growth.  When  that  is  abundant,  popu- 
lation may  become  dense,  and  social  activities  multifarious  and 
complicated,  while  a  scanty  vegetation  entails  sparse  population 
and  enfeebled  social  action.  Any  universal  disturbance  of  the 
physical  forces,  as  excessive  rains  or  drouth,  by  reducing  the  har- 


COKKELATION   OF   VITAL   AND   SOCIAL   FOKCES.     XXXV11 

vest,  is  felt  throughout  the  entire  social  organism.  Where  this 
effect  is  marked,  and  not  counteracted  by  free  communication  with 
more  fertile  regions,  the  means  of  the  community  become  restricted, 
business  declines,  manufactures  are  reduced,  trade  slackens,  travel 
falls  off,  luxuries  are  diminished,  education  is  neglected,  marriages 
are  fewer,  and  a  thousand  kindred  results  indicate  decline  of  enter- 
prise and  depression  of  the  social  energies. 

In  a  dynamical  point  of  view  there  is  a  strict  analogy  between 
the  individual  and  the  social  economies — the  same  law  of  force 
governs  the  development  of  both.  In  the  case  of  the  individual, 
the  amount  of  energy  which  he  possesses  at  any  time  is  limited, 
and  when  consumed  for  one  purpose  it  cannot  of  course  be  had  for 
another.  An  undue  demand  in  one  direction  involves  a  corre- 
sponding deficiency  elsewhere.  For  example,  excessive  action  of 
the  digestive  system  exhausts  the  muscular  and  cerebral  systems, 
while  excessive  action  of  the  muscular  system  is  at  the  expense  of 
the  cerebral  and  digestive  organs ;  and  again,  excessive  action  of 
the  brain  depresses  the  digestive  and  muscular  energies.  If  the 
fund  of  power  in  the  growing  constitutions  of  children  is  overdrawn 
in  any  special  channel,  as  is  often  the  case  by  excessive  stimulation 
of  the  brain,  the  undue  abstraction  of  energy  from  other  portions 
of  the  system  is  sure  to  entail  some  form  of  physiological  disaster. 
So  with  the  social  organism ;  its  forces  being  limited,  there  is  but  a 
definite  amount  of  power  to  be  consumed  in  the  various  social 
activities.  Its  appropriation  in  one  way  makes  impossible  its  em- 
ployment in  another,  and  it  can  only  gain  power  to  perform  one 
function  by  the  loss  of  it  in  other  directions.  This  fact,  that  social 
force  cannot  be  created  by  enactment,  and  that  when  dealing  with 
the  producing,  distributing,  and  commercial  activities  of  the  com- 
munity, legislation  can  do  little  more  than  interfere  with  their 
natural  courses,  deserves  to  be  more  thoroughly  appreciated  by  the 
public. 

But  the  law  in  question  has  yet  higher  bearings.  More  and 
more  we  are  perceiving  that  the  condition  of  humanity  and  tho 


XXXV1U  INTRODUCTION. 

progress  of  civilization  are  direct  resultants  of  the  forces  by  which 
men  are  controlled.  "What  we  term  the  moral  order  of  society,  im- 
plies a  strict  regularity  in  the  action  of  these  forces.  Modern  sta- 
tistics disclose  a  remarkable  constancy  in  the  moral  activities  man- 
ifested in  communities  of  men.  Crimes,  and  even  the  modes  of 
crime,  have  been  observed  to  occur  with  a  uniformity  which  admits 
of  their  prediction.  Each  period  may  therefore  be  said  to  have  its 
definite  amount  of  morality  and  justice.  It  has  been  maintained, 
for  instance,  with  good  reason,  that  "  the  degree  of  liberty  a  peo- 
ple is  capable  of  in  any  given  age,  is  a  fixed  quantity,  and  that  any 
artificial  extension  of  it  in  one  direction  brings  ab.out  an  equiva- 
lent limitation  in  some  other  direction.  French  revolutions  show 
scarcely  any  more  respect  for  individual  rights  than  the  despotisms 
they  supplant;  and  French  electors  use  their  freedom  to  put 
themselves  again  in  slavery.  So  in  those  communities  where  State 
restraint  is  feeble,  we  may  expect  to  find  it  supplemented  by  the 
sterner  restraints  of  public  opinion." 

But  society  like  the  individual  is  progressive.  Although  at 
each  stage  of  individual  growth  the  forces  of  the  organism,  physi- 
ological, intellectual,  and  passional,  have  each  a  certain  definite 
amount  of  strength,  yet  these  ratios  are  constantly  changing,  and 
it  is  in  this  change  that  development  essentially  consists.  So  with 
society;  the  measured  action  of  its  forces  gives  rise  to  a  fixed 
amount  of  morality  and  liberty  in  each  age,  but  that  amount  in- 
creases with  social  evolution.  The  savage  is  one  in  whom  certain 
classes  of  feelings  and  emotions  predominate,  and  he  becomes  civil- 
ized just  in  proportion  as  these  feelings  are  slowly  replaced  by  oth- 
ers of  a  higher  character.  Yet  the  activities  which  determine 
human  advancement  are  various.  Not  only  must  we  regard  the 
physiological  forces,  or  those  which  pertain  to  man's  physical  or- 
ganization and  capacities,  and  the  psychological,  or  those  resulting 
from  his  intellectual  and  emotional  constitution,  but  the  influences 
of  the  external  world,  and  those  of  the  social  state,  are  likewise  to 
be  considered.  Man  and  society,  therefore,  aa  viewed  by  the  eyo 


SPENCER'S  CONTRIBUTION  TO  THE  INQUIRY,     xxxix 

of  science,  present  a  series  of  vast  and  complex  dynamical  problems, 
which  are  to  be  studied  in  the  future  in  the  light  of  the  great 
law  by  which,  we  have  reason  to  believe,  all  forms  and  phases  of 
force  aro  governed. 

A  further  aspect  of  the  subject  remains  still  to  be  noticed.  Mr. 
Herbert  Spencer  has  the  honor  of  crowning  this  sublime  inquiry  by 
showing  that  the  law  of  the  conservation,  or  as  he  prefers  to  term  it 
the  '  Persistence  of  Force,'  as  it  is  the  underlying  principle  of  all  be- 
ing, is  also  the  fundamental  truth  of  all  philosophy.  With  masterly 
analytic  skill  he  has  shown  that  this  principle  of  which  the  human 
mind  has  just  become  fully  conscious,  is  itself  the  profoundest  law 
of  the  human  mind,  the  deepest  foundation  of  consciousness.  He 
has  demonstrated  that  the  law  of  the  Persistence  of  Force,  of  which 
the  most  piercing  intellects  of  past  times  had  but  partial  and  un- 
satisfying glimpses,  and  which  the  latest  scientific  research  has 
disclosed  as  a  great  principle  of  nature,  has  a  yet  more  transcendent 
character ;  is,  in  fact,  an  d  priori  truth  of  the  highest  order — a 
truth  which  is  necessarily  involved  in  our  mental  organization ; 
which  is  broader  than  any  possible  induction,  and  of  higher  validity 
than  any  other  truth  whatever.  This  principle,  which  is  at  once 
the  highest  result  of  scientific  investigation  and  metaphysical 
analysis,  Mr.  Spencer  has  made  the  basis  of  his  new  and  compre- 
hensive System  of  Philosophy ;  and  in  the  first  work  of  the  series, 
entitled  "  First  Principles,  "  he  has  developed  the  doctrine  in  its 
broadest  philosophic  aspects.  The  lucid  reasoning  by  which  he 
reaches  his  conclusions  cannot  be  presented  here ;  a  brief  extract 
or  two  will,  however,  serve  to  indicate  the  important  place  assigned 
to  the  law  by  this  acute  and  profound  inquirer : 

"  We  might,  indeed,  be  certain,  even  in  the  absence  of  any  such 
analysis  as  the  foregoing,  that  there  must  exist  some  principle 
which,  as  being  the  basis  of  science,  cannot  be  established  by  sci- 
ence. All  reasoned  out  conclusions  whatever  must  rest  on  some 
postulate.  As  before  shown,  we  cannot  go  on  merging  derivative 
truths  in  these  wider  and  wider  truths  from  which  they  are  de- 


Iff-} 


ffl 


INTEODUCTION. 


rived,  without  reaching  at  last  a  widest  truth  which  can  be  merged 
in  no  other,  or  derived  from  no  other.    And  whoever  contemplates 
;  the  relation  in  which  it  stands  to  the  truths  of  science  in  general, 
\r-will  see  that  this  truth,  transcending  demonstration,  is  the  Persist- 
^ence  of  force."  *  *  * 

'**  Such,  then,  is  the  foundation  of  any  possible  system  of  posi- 
tive knowledge.    Deeper  than  demonstration — deeper  even  than 
definite  cognition — deep  as  the  very  nature  of  mind,  is  the  postu- 
late at  which  we  have  arrived.    Its  authority  transcends  all  others 
-  whatever ;  for  not  only  is  it  given  in  the  constitution  of  our  own 
;  consciousness,  but  it  is  impossible  to  imagine  a  consciousness  so 
constituted  as  not  to  give  it.    Thought,  involving  simply  the  estab- 
lishment-of  relations,  may  be  readily  conceived  to  go  on  while  yet 
**o  these  relations  have  not  been  organized  into  the  abstracts  we  call 
space  and  time ;  and  so  there  is  a  conceivable  kind  of  consciousness 
(  which  does  not  contain  the  truths  commonly  called  d  priori,  in- 
0  volved  in  the  organization  of  these  forms  of  relations.    But  thought 
cannot  be  conceived  to  go  on  without  some  element  between  which 
its  relations  may  be  established ;  and  so  there  is  no  conceivable 
kind-  of  consciousness  which  does  not  imply  continued  existence  as 
ts  datum.     Consciousness  without  this  or  that  particular  form  is 
ible ;  but  consciousness  without  contents  is  impossible. 
"  The  sole  truth  which  transcends  experience  by  underlying  it,  is 
thus  the  Persistence  of  force.     This  being  the  basis  of  experience, 
must  be  the  basis  of  any  scientific  organization  of  experiences.     To 
this  an  ultimate  analysis  brings  us  down ;  and  on  this  a  rational 
synthesis  must  be  built  up." 

To  the  question,  What  then  is  the  value  of  experimental  inves- 
tigations upon  the  subject,  if  the  truth  sought  cannot  be  estab- 
lished by  inductions  from  them ?  Mr.  Spencer  replies:  "They  are 
of  value  as  disclosing  the  many  particular  implications  which  the 
general  truth  does  not  specify ;  they  are  of  value  as  teaching  us  how 
much  of  one  mode  of  force  is  the  equivalent  of  so  much  of  another 
mode ;  they  are  of  value  as  determining  uncler  what  conditions  each 


STUPENDOUS   REACH   OF   THE   LAW.  x 

metamorphosis  occurs ;  and  they  are  of  value  as  leading  us  to  in- 
quire in  what  shape  the  remnant  of  force  has  escaped,  when  the 
apparent  results  are  not  equivalent  to  the  cause."  And  it  may  be 
added,  that  it  is  to  these  investigations  that  we  are  indebted  for  the 
clear  and  comprehensive  establishment  of  the  principle  as  a  law  of 
physical  nature ;  psychological  analysis  having  only  shown  that  it 
extends  much  further  than  it  is  the  business  of  experimental  science 
to  go. 

Thus  the  law  characterized  by  Faraday  as  the  highest  in  phys- 
ical science  which  our  faculties  permit  us  to  perceive,  has  a  far 
more  extended  sway;  it  might  well  have  been  proclaimed  the 
highest  'law  of  all  science — the  most  far-reaching  principle  that 
adventuring  reason  has  discovered  in  the  universe.  Its  stupendous 
reach  spans  all  orders  of  existence.  Not  only  does  it  govern  the 
movements  of  the  heavenly  bodies,  but  it  presides  over  the  genesis 
of  the  constellations ;  not  only  does  it  control  those  radiant  floods 
of  power  which  fill  the  eternal  spaces,  bathing,  warming,  illumining 
and  vivifying  our  planet,  but  it  rules  the  actions  and  relations  of 
men,  and  regulates  the  march  of  terrestrial  affairs.  Nor  is  its  do- 
minion limited  to  physical  phenomena ;  it  prevails  equally  in  the 
world  of  mind,  controlling  all  the  faculties  and  processes  of  thought 
and  feeling.  The  star-suns  of  the  remoter  galaxies  dart  their  ra- 
diations across  the  universe ;  and  although  the  distances  are  so  pro- 
found that  hundreds  of  centuries  may  have  been  required  to  traverse 
them,  the  impulses  of  force  enter  the  eye,  and  impressing  an  atomic 
change  upon  the  nerve,  give  origin  to  the  sense  of  sight.  Star 
and  nerve-tissue  are  parts  of  the  same  system — stellar  and  nervous 
forces  are  correlated.  Nay  more ;  sensation  awakens  thought  and 
kindles  emotion,  so  that  this  wondrous  dynamic  chain  binds  into 
living  unity  the  realms  of  matter  and  mind  through  measureless 
amplitudes  of  space  and  time. 

And  if  these  high  realities  are  but  faint  and  fitful  glimpses 
which  science  has  obtained  in  the  dim  dawn  of  discovery,  what 
must  be  the  glories  of  the  coming  day  ?  If  indeed  they  are  but 


Xlii  INTRODUCTION. 

*  pebbles '  gathered  from  the  shores  of  the  great  ocean  of  truth, 
what  are  the  mysteries  still  hidden  in  the  bosom  of  tke  mighty  un- 
explored ?  And  how  far  transcending  all  stretch  of  thought  that 
Unknown  and  Infinite  Cause  of  all  to  which  the  human  spirit  turns 
evermore  in  solemn  and  mysterious  worship ! 

It  remains  only  to  observe,  that  so  immense  a  step  in  the  pro- 
gress of  our  knowledge  of  natural  agencies  as  the  following  pages 
disclose,  cannot  be  without  effect  upon  the  intellectual  culture 
of  the  age.  To  the  adherents  of  that  scholastic  and  verbal  edu- 
cation which  prefers  words  to  things,  and  ancient  to  modern 
thought ;  which  ignores  the  study  of  nature,  and  regards  the  pro- 
gress of  science  with  indifference  or  hostility,  it  matters  little  what 
views  of  the  world  are  entertained,  or  what  changes  these  views 
may  undergo.  But  there  is  another,  and  happily  an  increasing  class, 
who  hold  that  it  is  the  true  destiny  of  mind  to  comprehend  the 
vast  order  of  existence  in  the  midst  of  which  it  is  placed,  and  that 
the  faculties  of  man  are  divinely  adapted  to  this  sublime  task ;  who 
see  that  the  laws  of  nature  must  be  understood  before  they  can  be 
obeyed,  and  that  only  through  this  understanding  can  man  rise  to 
the  mastery  of  its  powers,  and  bring  himself  into  final  harmony 
with  his  conditions.  These  will  recognize  that  the  discovery  of 
new  principles  which  expand,  and  elevate,  and  harmonize  our  views 
of  the  universe — which  involve  the  workings  of  the  mind  itself, 
open  a  new  chapter  in  philosophy,  and  touch  the  very  foundations 
of  knowledge,  cannot  be  without  a  determining  influence  upon  tho 
future  course  and  development  of  thought,  and  the  spirit  and 
methods  of  its  acquisition. 


T  ts**—    UT  c  cc^te^L-, 


^^x^-, 


THE 

CORRELATION 


OF   PHYSICAL    FOEOES 

BY  W.  R.  GROVE,  Q.C.,  M.A.,  P.R.S. 

FIEST  AMERICAN,  FEOM  THE  FOURTH  ENGLISH  EDITION. 


WILLIAM  ROBERT  GROVE,  an  English  lawyer  and  physicist,  was  born  at 
Swansea,  July  14,  1811.  He  graduated  at  Oxford  in  1834,  and  during  the 
next  five  years  was  Professor  of  Natural  Philosophy  at  the  London  Insti- 
tution. Professor  Grove  is  a  rare  example  of  the  ability  which  has  achieved 
a  distinguished  eminence  in  different  fields  of  effort.  While  pursuing  with 
marked  success  the  profession  of  an  advocate,  he  has  devoted  his  leisure  to 
original  scientific  researches,  and  obtained  a  high  distinction  both  as  a  dis- 
coverer and  a  philosophic  writer  upon  scientific  subjects.  In  1862  he  was 
made  Queen's  Counsel,  and  afterwards  Vice-President  of  the  Royal  Society. 
He  is  the  inventor  of  the  powerful  galvanic  battery  known  by  his  name,  and 
his  chief  researches  have  been  in  the  field  of  electricity.  Many  of  his  ex- 
perimental results  are  referred  to  in  the  following  pages,  which  will  also 
attest  his  high  position  among  the  founders  of  the  new  philosophy  of 
forces. 


PREFACE. 


THE  Plirase  '  Correlation  of  Physical  Forces '  in  the  sense  in 
which  I  have  used  it,  having  become  recognized  by  a  large 
number  of  scientific  writers,  it  would  produce  confusion  were  I 
now  to  adopt  another  title.  It  would,  perhaps,  have  been  better 
if  I  had  in  the  first  instance  used  the  term  Co-relation,  as  the 
words  '  correlate,'  *  correlative,'  had  acquired  a  peculiar  metaphys- 
ical sense  somewhat  differing  from  that  which  I  attached  to  the 
substantive  correlation.  The  passage  in  the  text  (p.  183)  explains 
the  meaning  I  have  given  to  the  term. 

Twenty  years  having  elapsed  since  I  promulgated  the  views 
contained  in  this  Essay,  which  were  first  advanced  in  a  lecture  at 
the  London  Institution  in  January  1842,  and  subsequently  more 
fully  developed  in  a  course  of  lectures  in  1843, 1  think  it  advisable 
to  add  a  little  to  the  Preface  with  reference  to  other  labourers  in 
the  same  field. 

It  has  happened  with  this  subject  as  with  many  others,  that 
similar  ideas  have  independently  presented  themselves  to  differ- 
ent minds  about  the  Same  period.  In  May  1842  a  paper  was 
published  by  M.  Mayer  which  I  had  not  read  when  my  last  edition 
was  published,  and  indeed  only  now  know  imperfectly  by  the 
viva-wee  translation  of  a  friend.  It  deduces  very  much  the  same 
conclusions  to  which  I  had  been  led,  the  author  starting  partly 
from  d  priori  reasoning  and  partly  from  an  experiment  by  which 
water  was  heated  by  agitation,  and  from  another,  which  had,  how- 
ever, previously  been  made  by  Davy,  viz.  that  ice  can  be  melted 
by  friction,  though  kept  in  a  medium  which  is  below  the  freezing 
point  of  water. 

In  1843  a  paper  by  Mr.  Joule  on  the  mechanical  equivalent  of 


4:  PEEFACE. 

heat  appeared,  which,  though  not  in  terms  touching  on  the  mutual 
and  necessary  dependence  of  all  the  Physical  Forces,  yet  bears 
most  importantly  upon  the  doctrine. 

While  my  third  edition  was  going  through  the  press  I  had 
the  good  fortune  to  make  the  acquaintance  of  M.  Seguin,  who 
informed  me  that  his  uncle,  the  eminent  Montgolfier,  had  long 
entertained  the  idea  that  force  was  indestructible,  though,  with 
the  exception  of  one  sentence,  in  his  paper  on  the  hydraulic  ram, 
and  where  he  is  apparently  speaking  of  mechanical  force,  he  has 
left  nothing  in  print  on  the  subject.  Not  so,  however,  M.  Seguin 
himself,  who  in  1839,  in  a  work  on  the  '  Influence  of  Railroads,' 
has  distinctly  expressed  his  uncle's  and  his  own  views  on  the 
identity  of  heat  and  mechanical  force,  and  has  given  a  calculation 
of  their  equivalent  relation,  which  is  not  far  from  the  more  recent 
numerical  results  of  Mayer,  Joule,  and  others. 

Several  of  the  great  mathematicians  of  a  much  earlier  period 
advocated  the  idea  of  what  they  termed  the  Conservation  of  Force, 
but  although  they  considered  that  a  body  in  motion  would  so 
continue  for  ever,  unless  arrested  by  the  impact  of  another  body, 
and,  indeed,  in  the  latter  case,  would,  if  elastic,  still  continue  to 
move  (though  deflected  from  its  course)  with  a  force  proportion- 
ate to  its  elasticity,  yet  with  inelastic  bodies  the  general,  and,  as 
far  as  I  am  aware,  the  universal  belief  was,  that  the  motion  was 
arrested  and  the  force  annihilated.  Montgolfier  went  a  step  far- 
ther, and  his  hydraulic  ram  was  to  him  a  proof  of  the  truth  of 
his  preconceived  idea,  that  the  shock  or  impact  of  bodies  left  the 
mechanical  force  undestroyed. 

Previously,  however,  to  the  discoveries  of  the  voltaic  battery, 
electro-magnetism,  thermo-electricity,  and  photography,  it  was 
impossible  for  any  mind  to  perceive  what,  in  the  greater  number 
of  cases,  became  of  the  force  which  was  apparently  lost.  The 
phenomena  of  heat,  known  from  the  earliest  times,  would  have 
been  a  mode  of  accounting  for  the  resulting  force  in  many  cases 
where  motion  was  arrested,  and  we  find  Bacon  announcing  a 
theory  that  motion  was  the  form,  as  he  quaintly  termed  it,  of 
heat.  Rumford  and  Davy  adopted  this  view,  the  former  with  a 
fair  approximate  attempt  at  numerical  calculation,  but  no  one  of 
these  philosophers  seems  to  have  connected  it  with  the  inde- 
structibility of  force.  A  passage  in  the  writings  of  Dr.  Roget, 


PREFACE.  O 

combating  the  theory  that  mere  contact  of  dissimilar  bodies  was 
the  source  of  voltaic  electricity,  philosophically  supports  his  argu- 
ment by  the  idea  of  non-creation  of  force. 

As  I  have  introduced  into  the  later  editions  of  my  Essay  ab- 
stracts of  the  different  discoveries  which  I  have  found,  since  my 
first  lectures,  to  bear  upon  the  subject,  I  have  been  regarded  by 
many  rather  as  the  historian  of  the  progress  made  in  this  branch 
of  thought  than  as  one  who  has  had  anything  to  do  with  its  ini- 
tiation. Everyone  is  but  a  poor  judge  where  he  is  himself  inter- 
ested, and  I  therefore  write  with  diffidence,  but  it  would  be  affect- 
ing an  indifference  which  I  do  not  feel  if  I  did  not  state  that  I 
believe  myself  to  have  been  the  first  who  introduced  this  subject 
as  a  generalised  system  of  philosophy,  and  continued  to  enforce 
it  in  my  lectures  and  writings  for  many  years,  during  which  it 
met  with  the  opposition  usual  and  proper  to  novel  ideas. 

Avocations  necessary  to  the  well-being  of  others  have  prevent- 
ed my  following  it  up  experimentally,  to  the  extent  that  I  once 
hoped ;  but  I  trust  and  believe  that  this  Essay,  imperfect  though 
it  be,  has  helped  materially  to  impress  on  that  portion  of  the 
public  which  devotes  its  attention  rather  to  the  philosophy  of 
science  than  to  what  is  now  termed  science,  the  truth  of  the  thesis 
advocated. 

To  show  that  the  work  of  to-day  is  not  substantially  different 
from  the  thoughts  I  first  published  on  the  subject,  at  a  period 
when  I  knew  little  or  nothing  of  what  had  been  thought  before, 
I  venture  to  give  a  few  extracts  from  the  printed  copy  of  my 
lecture  of  1842  :— 

Physical  Science  treats  of  Matter,  and  what  I  shall  to-night  terra  its 
Affections ;  namely,  Attraction,  Motion,  Heat,  Light,  Electricity,  Magnet- 
ism, Chemical-Affinity.  When  these  re-act  upon  matter,  they  constitute 
Forces.  The  present  tendency  of  theory  seems  to  lead  to  the  opinion  that 
all  these  Affections  are  resolvable  into  one,  namely,  Motion;  however, 
should  the  theories  on  these  subjects  be  ultimately  so  effectually  gener- 
alised as  to  become  laws,  they  cannot  avoid  the  necessity  for  retaining  dif- 
ferent names  for  these  different  Affections ;  or,  as  they  would  then  be  called, 
different  modes  of  Motion 

(Ersted  proved  that  Electricity  and  Magnetism  are  two  forces  which  act 
upon  each  other ;  not  in  straight  lines,  as  all  other  known  forces  do,  but  in 
a  rectangular  direction :  that  is,  that  bodies  invested  with  electricity,  or  the 
conduits  of  an  electric-current,  tend  to  place  magnets  at  right  angles  to 
them ;  and,  conversely,  that  magnets  teud  to  place  bodies  conducting  elec- 
tricity at  right  angles  to  them 


6  PREFACE. 


The  discovery  of  (Ersted,  by  which  electricity  was  made  a  source  of 
Magnetism,  soon  led  philosophers  to  seek  the  converse  effect ;  that  is,  to 
educe  Electricity  from  a  permanent  magnet : — had  these  experimentalists 
succeeded  in  their  expectations  of  making  a  stationary  magnet  a  source  of 
electric-currents,  they  would  have  realised  the  ancient  dreams  of  perpetual 
motion,  they  would  have  converted  statics  into  dynamics,  they  would  have 
produced  power  without  expenditure ;  in  other-words,  they  would  have  be- 
come creators.  They  failed,  and  Faraday  saw  their  error ;  he  proved  that 
to  obtain  Electricity  from  Magnetism  it  was  necessary  to  superadd  to  this 
latter,  motion;  that  magnets  while  in  motion  induced  electricity  in  con- 
tiguous conductors ;  and  that  the  direction  of  such  electric-currents  was 
tangential  to  the  polar  direction  of  the  magnet ;  that  as  Dynamic-electricity 
may  be  made  the  source  of  Magnetism  and  Motion,  so  Magnetism  conjoined 
with  Motion  may  be  made  the  source  of  Electricity.  Here  originates  the 
Science  of  Magneto-electricity,  the  true  converse  of  Electro-magnetism; 
and  thus  between  Electricity  and  Magnetism  is  shown  to  exist  a  reciprocity 
of  force  such  that,  considering  either  as  the  primary  agent,  the  other  be- 
comes the  re-agent ;  viewing  one  in  the  relation  of  cause,  the  other  is  the 
effect 

The  Science  of  Thermo-Electricity  connected  heat  with  electricity,  and 
proved  these,  like  all  other  natural  forces,  to  be  capable  of  mutual  reac- 
tion  

Voltaic  action  is  Chemical  action  taking  place  at  a  distance  or  trans- 
ferred through  a  chain  of  media ;  and  the  Daltonian  equivalent  numbers 
are  the  exponents  of  the  amount  of  voltaic  action  for  corresponding  chemi- 
cal substances.  .  .  . 

By  regarding  the  quantity  of  electrical,  as  directly  proportional  to  the 
efficient  chemical  action,  and  by  experimentally  tracing  this  principle,  I 
have  been  fortunate  enough  to  increase  the  power  of  the  Voltaic-pile 
more  than  sixteen  times,  as  compared  with  any  combination  previously 
known 

I  am  strongly  disposed  to  consider  that  the  facts  of  Catalysis  depend 
upon  voltaic  action,  to  generate  which  three  heterogeneous  substances  are 
always  necessary.  Induced  by  this  belief  I  made  some  experiments  on  the 
subject,  and  succeeded  in  forming  a  voltaic  combination  by  gaseous-oxygen, 
gaseous-hydrogen,  and  platinum ;  by  which  a  galvanometer  was  deflected 
and  water  decomposed 

It  appears  to  me  that  heat  and  light  may  be  considered  as  affections ; 
or,  according  to  the  Undulatory-theory,  vibrations  of  matter  itself,  and  not 
of  a  distinct  etherial  fluid  permeating  it :  these  vibrations  would  be  prop- 
agated, just  as  sound  is  propagated  by  vibrations  of  wood  or  as  waves  by 
water.  To  my  mind,  all  the  consequences  of  the  Undulatory-theory  flow 
as  easily  from  this,  as  from  the  hypothesis  of  a  specific  ether ;  to  suppose 
which,  namely,  to  suppose  a  fluid  sui  generis,  and  of  extreme  tenuity  pene- 
trating solid  bodies,  we  must  assume,  first,  the  existence  of  the  fluid  itself; 
secondly,  that  bodies  are  without  exception  porous ;  thirdly,  that  these 
pores'  communicate;  fourthly,  that  matter  is  limited  in  expansibility. 
None  of  these  difficulties  apply  to  the  modification  of  this  theory  which  I 
venture  to  propose ;  and  no  other  difficulty  applies  to  it  which  does  not 
equally  apply  to  the  received  hypothesis.  With  regard  to  the  planetary 
spaces,  the  diminishing  periods  of  comets  is  a  strong  argument  for  the  ex- 
istence of  an  universally-diffused  matter :  this  has  the  function  of  resist- 


PREFACE.  7 

ance,  and  there  appears  to  be  no  reason  to  divest  it  of  the  functions  com- 
mon to  all  matter,  or  superficially  to  appropriate  it  to  certain  affections. 
Again,  the  phenomena  of  transparency  and  opacity  are,  to  my  mind,  more 
easily  explicable  by  the  former  than  by  the  latter  theory ;  as  resulting  from 
a  difference  in  the  molecular  arrangement  of  the  matter  affected.  In  re- 
gard to  the  effects  of  double-refraction  and  polarisation,  the  molecular 
gives  at  once  a  reason  for  the  effects  upon  the  one  theory,  while  upon  the 
other  we  must,  in  addition  to  previous  assumptions,  further  assume  a  dif- 
ferent elasticity  of  the  ether  in  different  directions  within  the  doubly- 
refracting  medium.  The  same  theory  is  applicable  to  Electricity  and 
Magnetism ;  my  own  experiments  on  the  influence  of  the  elastic  intermedium 
on  the  voltaic-arc,  and  those  of  Faraday  on  electrical  induction,  furnish 
strong  arguments  in  support  of  it.  My  inclination  would  lead  me  to  de- 
tain you  on  this  subject  much  longer  than  my  judgment  deems  advisable : 
I  therefore  content  myself  with  offering  it  to  your  consideration,  and, 
should  my  avocations  permit,  I  may  at  a  future  period  more  fully  develope 

it 

Light,  Heat,  Electricity,  Magnetism,  Motion,  and  Chemical-affinity,  are 
all  convertible  material  affections ;  assuming  either  as  the  cause,  one  of 
the  others  will  be  the  effect :  thus  heat  may  be  said  to  produce  electricity, 
electricity  to  produce  heat ;  magnetism  to  produce  electricity,  electricity 
magnetism;  and  so  of  the  rest.  Cause  and  effect,  therefore,  in  their -ab- 
stract relation  to  these  forces,  are  words  solely  of  convenience:  we  are 
totally  unacquainted  with  the  ultimate  generating  power  of  each  and  all 
of  them,  and  probably  shall  ever  remain  so ;  we  can  only  ascertain  the 
normae  of  their  action :  we  must  humbly  refer  their  causation  to  one  omni- 
present influence,  and  content  ourselves  with  studying  their  effects  and 
developing  by  experiment  their  mutual  relations. 

I  have  transposed  the  passages  relating  to  voltaic  action  and 
catalysis,  but  I  have  not  added  a  word  to  the  above  quotation, 
and,  as  far  as  I  am  now  aware,  the  theory  that  the  so-called  im- 
ponderables are  affections  of  ordinary  matter,  that  they  are  re- 
solvable into  motion,  that  they  are  to  be  regarded  in  their  action 
on  matter  as  forces,  and  not  as  specific  entities,  and  that  they  are 
capable  of  mutual  reaction,  thence  alternately  acting  as  cause  and 
effect,  had  not  at  that  time  been  publicly  advanced. 

My  original  Essays  being  a  record  of  lectures,  and  being  pub- 
lished by  the  managers  of  the  Institution,  I  necessarily  adhered 
to  the  form  and  matter  which  I  had  orally  communicated.  In 
preparing  subsequent  editions  I  found  that,  without  destroying 
the  identity  of  the  work,  I  could  not  alter  the  style  ;  although  it 
would  have  been  less  difficult  and  more  satisfactory  to  me  to  have 
done  so,  the  work  would  not  have  been  a  republication ;  and  I 
was  for  obvious  reasons  anxious  to  preserve  as  far  as  I  could  the 
original  text,  which,  though  added  to,  is  but  little  altered. 

The  form  of  lectures  has  necessarily  continued  the  use  of  the 


8  PREFACE. 

first  person,  and  I  would  beg  my  readers  not  to  attribute  to  me, 
from  the  modes  of  expression  used,  a  dogmatism  which  is  far  from 
my  thought.  If  my  opinions  are  expressed  broadly,  it  is  that,  if 
opinions  are  always  hedged  in  by  qualifications,  the  style  becomes 
embarrassed  and  the  meaning  frequently  unintelligible. 

As  a  course  of  lectures  can  only  be  useful  by  inducing  the 
auditor  to  consult  works  on  the  subject  he  hears  treated,  so  the 
object  of  this  Essay  is  more  to  induce  a  particular  train  of  thought 
on  the  known  facts  of  physical  science  than  to  enter  with  minute 
criticisms  into  each  separate  branch. 

In  one  or  two  of  the  reviews  of  previous  editions  the  general 
idea  of  the  work  was  objected  to.  I  believe,  however,  that  will 
not  now  be  the  case ;  the  mathematical  labours  of  Mr.  Thompson, 
Clausius,  and  others,  though  not  suitable  for  insertion  in  an 
Essay  such  as  this,  have  awakened  an  interest  for  many  portions 
of  the  subject,  which  promises  much  for  its  future  progress. 

The  short  and  irregular  intervals  which  my  profession  permits 
me  to  devote  to  science  so  prevent  the  continuity  of  attention 
necessary  for  the  proper  evolution  of  a  train  of  thought,  that  I 
certainly  should  not  now  have  courage  to  publish  for  the  first 
time  such  an  Essay ;  and  it  is  only  the  favour  it  has  received 
from  those  whose  opinions  I  highly  value,  and  the,  I  trust  pardon- 
able, wish  not  to  let  some  favourite  thoughts, of  my  youth  lose  all 
connection  with  my  name,  that  have  induced  me  to  reprint  it. 

My  scientific  readers  will,  I  hope,  excuse  the  very  short  notices 
of  certain  branches  of  science  which  are  introduced,  as  without 
them  the  work  would  be  unintelligible  to  many  for  whom  it  is 
intended.  I  have  endeavoured  so  to  arrange  my  matter  that  each 
division  should  form  an  introduction  to  those  which  follow,  and 
to  assume  no  more  preliminary  knowledge  to  be  possessed  by  my 
readers  than  would  be  expected  from  persons  acquainted  with  the 
elements  of  physical  science. 

The  notes  contain  references  to  the  original  memoirs  in  which 
the  branches  of  science  alluded  to  are  to  be  found,  as  well  as  to 
those  which  bear  on  the  main  arguments ;  where  these  memoirs 
are  numerous,  or  not  easy  of  access,  I  have  referred  to  treatises  in 
which  they  are  collated.  To  prevent  the  reader's  attention  being 
interrupted,  I  have  in  the  notes  referred  to  the  pages  of  the  text, 
instead  of  to  interpolated  letters. 


COKKELATION 

OF 

PHYSICAL   FOEOES. 


L— INTRODUCTORY  REMARKS. 

WHEN  natural  phenomena  are  for  the  first  time  ob- 
served, a  tendency  immediately  developes  itself  to 
refer  them  to  something  previously  known — to  bring  them 
within  the  range  of  acknowledged  sequences.  The  mode  of 
regarding  new  facts,  which  is  most  favourably  received  by 
the  public,  is  that  which  refers  them  to  recognised  views — 
stamps  them  into  the  mould  in  which  the  mind  has  been  al- 
ready shaped.  The  new  fact  may  be  far  removed  from  those 
to  which  it  is  referred,  and  may  belong  to  a  different  order 
of  analogies,  but  this  cannot  then  be  known,  as  its  co-ordi- 
nates are  wanting.  It  may  be  questionable  whether  the 
mind  is  not  so  moulded  by  past  events  that  it  is  impossible 
to  advance  an  entirely  new  view,  but  admitting  such  possi- 
bility, the  new  view,  necessarily  founded  on  insufficient  data, 
is  likely  to  be  more  incorrect  and  prejudicial  than  even  a 
strained  attempt  to  reconcile  the  new  discovery  with  known 
facts. 

The  theory  consequent  upon  new  facts,  whether  it  be  a 
co-ordination  of  them  with  known  ones,  or  the  more  difficult 
1* 


10  CORRELATION"  OF   PHYSICAL   FOECES. 

and  dangerous  attempt  at  remodelling  the  public  ideas,  is 
generally  enunciated  by  the  discoverers  themselves  of  the 
facts,  or  by  those  to  whose  authority  the  world  at  the  period 
of  the  discovery  defers ;  others  are  not  bold  enough,  or  if 
they  be  so,  are  unheeded.  The  earliest  theories  thus  enuncia- 
ted obtain  the  firmest  hold  upon  the  public  mind,  for  at  such 
a  time  there  is  no  power  of  testing,  by  a  sufficient  range  of 
experience,  the  truth  of  the  theory ;  it  is  accepted  solely  or 
mainly  upon  authority :  there  being  no  means  of  contradic- 
tion, its  reception  is,  in  the  first  instance,  attended  with  some 
degree  of  doubt,  but  as  *the  time  in  which  it  can  fairly  be  in- 
vestigated far  exceeds  that  of  any  lives  then  in  being,  and  as 
neither  the  individual  nor  the  public  mind  will  long  tolerate 
a  state  of  abeyance,  a  theory  shortly  becomes,  for  want 
of  a  better,  admitted  as  an  established  truth :  it  is  handed 
from  father  to  son,  and  gradually  takes  its  place  in  edu- 
cation. Succeeding  generations,  whose  minds  are  thus 
formed  to  an  established  view,  are  much  less  likely  to  aban- 
don it.  They  have  adopted  it  in  the  first  instance,  upon  au- 
thority, to  them  unquestionable,  and  subsequently  to  yield  up 
their  faith  would  involve  a  laborious  remodelling  of  ideas,  a 
task  which  the  public  as  a  body  will  and  can  rarely  under- 
take, the  frequent  occurrence  of  which  is  indeed  inconsistent 
with  the  very  existence  of  man  in  a  social  state,  as  it  would 
induce  an  anarchy  of  thought — a  perpetuity  of  mental  revo- 
lutions. 

This  necessity  has  its  good ;  but  the  prejudicial  effect 
upon  the  advance  of  science  is,  that  by  this  means,  theories  the 
most  immature  frequently  become  the  most  permanent ;  for 
no  theory  can  be  more  immature,  none  is  likely  to  be  so  in- 
correct, as  that  which  is  formed  at  the  first  flush  of  a  new 
discovery ;  and  though  time  exalts  the  authority  of  those 
from  whom  it  emanated,  time  can  never  give  to  the  illustri- 
ous dead  the  means  of  analysing  and  correcting  erroneous 
views  which  subsequent  discoveries  confer. 


INTRODUCTORY   REMARKS.  11 

Take  for  instance  the  Ptolemaic  System,  which  we  may 
almost  literally  explain  by  the  expression  of  Shakspeare : 
'  He  that  is  giddy  thinks  the  world  turns  round/  We  now 
see  the  error  of  this  system,  because  we  have  all  an  immedi- 
ate opportunity  of  refuting  it ;  but  this  identical  error  was 
received  as  a  truth  for  centuries,  because,  when  first  promul- 
gated, the  means  of  refuting  it  were  not  at  hand,  and  when 
the  means  of  its  refutation  became  attainable,  mankind  had 
been  so  educated  to  the  supposed  truth,  that  they  rejected  the 
proof  of  its  fallacy. 

I  have  premised  the  above  for  two  reasons  :  first  to  obtain 
a  fair  hearing,  by  requesting  as  far  as  possible  a  dismissal 
from  the  mind  of  my  readers  of  preconceived  views  by  and  in 
favour  of  which  all  are  liable  to  be  prejudiced ;  and  secondly, 
to  defend  myself  from  the  charge  of  undervaluing  authority, 
or  treating  lightly  the  opinions  of  those  to  whom  and  to 
whose  memory  mankind  looks  with  reverence.  Properly  to 
value  authority,  we  should  estimate  it  together  with  its  means 
of  information :  if '  a  dwarf  on  the  shoulders  of  a  giant  can 
see  further  than  the  giant/  he  is  no  less  a  dwarf  in  compari- 
son with  the  giant. 

The  subject  on  which  I  am  about  to  treat — viz.,  the  rela- 
tion of  the  affections  of  mattei  to  each  other  and  to  matter — 
peculiarly  demands  an  unprejudiced  regard.  The  different 
aspects  under  which  these  agencies  have  been  contemplated  ; 
the  different  views  which  have  been  taken  of  matter  itself ; 
the  metaphysical  subtleties  to  which  these  views  unavoidably 
lead,  if  pursued  beyond  fair  inductions  from  existing  expe- 
rience, present  difficulties  almost  insurmountable. 

The  extent  of  claim  which  my  views  on  this  subject  may 
have  to  originality  has  been  stated  in  the  Preface  ;  they  be- 
came strongly  impressed  upon  my  mind  at  a  period  when  I 
was  much  engaged  in  experimental  research,  and  were,  as  I 
then  believed,  and  still  believe,  regarding  them  as  a  system, 
new :  expressions  in  the  works  of  different  authors,  bearing 


12  COEKELATION   OF   PHYSICAL  FORCES. 

more  or  less  on  the  subject,  have  subsequently  been  pointed 
out  to  me,  some  of  which  go  back  to  a  distant  period.  An 
attempt  to  analyse  these  in  detail,  and  to  trace  how  far  I 
have  been  anticipated  by  others,  would  probably  but  little 
interest  the  reader,  and  in  the  course  of  it  I  should  constantly 
have  to  make  distinctions  showing  wherein  I  differed,  and 
wherein  I  agreed  with  others.  I  might  cite  authorities  which 
appear  to  me  to  oppose,  and  others  which  appear  to  coincide 
with  certain  of  the  views  I  have  put  forth ;  but  this  would 
interrupt  the  consecutive  developement  of  my  own  ideas,  and 
might  render  me  liable  to. the  charge  of  misconstruing  those  of 
others  ;  I  therefore  think  it  better  to  avoid  such  discussion  in 
the  text ;  and  in  addition  to  the  sketch  given  in  the  Preface, 
to  furnish  in  the  notes  at  the  conclusion  such  references 
to  different  authors  as  bear  upon  the  subjects  treated  of, 
which  I  have  discovered,  or  which  have  been  pointed  out  to 
me  since  the  delivery  of  the  lectures  of  which  this  essay 
is  a  record. 

The  more  extended  our  research  becomes,  the  more  we 
find  that  knowledge  is  a  thing  of  slow  progression,  that  the 
very  notions  which  appear  to  ourselves  new,  have  arisen, 
though  perhaps  in  a  very  indirect  manner,  from  successive 
modifications  of  traditional  opinions.  Each  word  we  utter, 
each  thought  we  think,  has  in  it  the  vestiges,  is  in  itself  the 
impress,  of  antecedent  words  and  thoughts.  As  each  ma- 
terial form,  could  we  rightly  read  it,  is  a  book,  containing  in 
itself  the  past  history  of  the  world  ;  so,  different  though  our 
philosophy  may  now  appear  to  be  from  that  of  our  progeni- 
tors, it  is  but  theirs  added  to  or  subtracted  from,  transmitted 
drop  by  arop  through  the  filter  of  antecedent,  as  ours  will  be 
through  that  of  subsequent,  ages. — The  relic  is  to  the  past  as 
is  the  germ  to  the  future. 

Though  many  valuable  facts,  and  correct  deductions  from 
them,  are  to  be  found  scattered  amongst  the  voluminous 
works  of  the  ancient  philosophers ;  yet,  giving  them  the 


INTRODUCTORY   KEMABKS.  13 

credit  which  they  pre-eminently  deserve  for  having  devoted 
their  lives  to  purely  intellectual  pursuits,  and  for  having 
thought,  seldom  frivolously,  often  profoundly,  nothing  can  be 
more  difficult  than  to  seize  and  apprehend  the  ideas  of  those 
who  reasoned  from  abstraction  to  abstraction — who,  although, 
as  we  now  believe,  they  must  have  depended  upon  observa- 
tion for  their  first  inductions,  afterwards  raised  upon  them 
such  a  complex  superstructure  of  syllogistic  deductions,  that, 
without  following  the  same  paths,  and  tracing  the  same  sinu- 
osities which  led  them  to  their  conclusions,  such  conclusions 
are  to  us  unintelligible.  To  think  as  another  thought,  we 
must  be  placed  in  the  same  situation  as  he  was  placed :  the 
errors  of  commentators  generally  arise  from  their  reasoning 
upon  the  arguments  of  their  text,  either  in  blind  obedience  to 
its  dicta,  without  considering  the  circumstances  under  which 
they  were  uttered,  or  in  viewing  the  images  presented  to  the 
original  writer  from  a  different  point  to  that  from  which  he 
viewed  them.  Experimental  philosophy  keeps  in  check  the 
errors  both  of  a  priori  reasoning  and  of  commentators,  and, 
at  all  events,  prevents  their  becoming  cumulative ;  though 
the  theories  or  explanations  of  a  fact  be  different,  the  fact 
remains  the  same.  It  is,  moreover,  itself  the  exponent  of  its 
discoverer's  thought :  the  observation  of  known  phenomena 
has  led  him  to  elicit  from  nature  the  new  phenomena :  and, 
though  he  may  be  wrong  in  his  deductions  from  this  after  its 
discovery,  the  reasonings  which  conducted  him  to  it  are  them 
selves  valuable,  and,  having  led  from  known  to  unknown 
truths,  can  seldom  be  uninstructive. 

Very  different  views  existed  amongst  the  ancients  as  to  the 
aims  to  be  pursued  by  physical  investigation,  and  as  to  the  objects 
likely  to  be  attained  by  it.  I  do  not  here  mean  the  moral  ob- 
jects, such  as  the  attainment  of  the  summum  lonum,  &c. 
— but  the  acquisitions  in  knowledge  which  such  investiga- 
tions were  likely  to  confer.  Utility  was  one  object  in  view, 
and  this  was  to  some  extent  attained  by  the  progress  m€de  in 


14:  CORRELATION   OF   PHYSICAL   FORCES. 

astronomy  and  mechanics  ;  Archimedes,  for  instance,  seems 
to  have  constantly  had  this  end  in  view  ;  but,  while  pursuing 
natural  knowledge  for  the  sake  of  knowledge  and  the  power 
which  it  brings  with  it,  the  greater  number  seemed  to  enter- 
tain an  expectation  of  arriving  at  some  ultimate  goal,  some 
point  of  knowledge,  which  would  give  them  a  mastery  over 
the  mysteries  of  nature,  and  would  enable  them  to  ascertain 
what  was  the  most  intimate  structure  of  matter,  and  the 
causes  of  the  changes  it  exhibits.  Where  they  could  not  dis- 
cover, they  speculated.  Leucippus,  Democritus,  and  others, 
have  given  us  their  notions  of  the  ultimate  atoms  of  which 
matter  was  formed,  and  of  the  modus  agendi  of  nature  in  the 
various  transformations  which  matter  undergoes. 

The  expectation  of  arriving  at  ultimate  causes  or  essences 
continued  long  after  the  speculations  of  the  ancients  had  been 
abandoned,  and  continues  even  to  the  present  day  to  be  a  very 
general  notion  of  the  objects  to  be  ultimately  attained  by 
physical  science.  Francis  Bacon,  the  great  remodeller  of 
science,  entertained  this  notion,  and  thought  that,  by  experi- 
mentally testing  natural  phenomena,  we  should  be  enabled  to 
trace  them  to  certain  primary  essences  or  causes  whence  the 
various  phenomena  flow.  These  he  speaks  of  under  the 
scholastic  name  of  '  forms  ' — a  term  derived  from  the  ancient 
philosophy,  but  differently  applied.  He  appears  to  have  un- 
derstood by  '  form '  the  essence  of  quality — that  in  which,  ab- 
stracting everything  extraneous,  a  given  quality  consists,  or 
that  which,  superinduced  on  any  body,  would  give  it  its  pe- 
culiar quality :  thus  the  form  of  transparency,  is  that 
which  constitutes  transparency,  or  that  by  which, .  when  dis- 
covered, transparency  could  be  produced  or  superinduced. 
To  take  a  specific  example  of  what  I  may  term  the  syn- 
thetic application  of  his  philosophy : — '  In  gold  there  meet 
together  yellowness,  gravity,  malleability,  fixedness  in  the  fire, 
a  determinate  way  of  solution,  which  are  the  simple  natures 
in  gold ;  for  he  who  understands  form,  and  the  manner  of 


INTRODUCTORY   REMARKS.  15 

superinducing  this  yellowness,  gravity,  ductility,  fixedness, 
faculty  of  fusion,  solution,  &c.,  with,  their  particular  degrees 
and  proportions,  will  consider  how  to  join  them  together  in 
some  body,  so  that  a  transmutation  into  gold  shall  follow/ 

On  the  other  hand,  the  analytic  method,  or,  '  the  enquiry 
from  what  origin  gold  or  any  other  metal  or  stone  is  generated 
from  its  first  fluid  matter  or  rudiments,  up  to  a  perfect  min- 
eral/ is  to  be  perceived  by  what  Bacon  calls  the  latent  pro- 
cess, or  a  search  for  c  what  in  every  generation  or  transfor- 
mation of  bodies,  flies  off,  what  remains  behind,  what  is  add- 
ed, what  separated,  &c. ;  also,  in  other  alterations  and  mo- 
tions, what  gives  motion,  what  governs  it,  and  the  like/ 
Bacon  appears  to  have  thought  that  qualities  separate  from 
the  substances  themselves  were  attainable,  and  if  not  capable 
of  physical  isolation,  were  at  all  events  capable  of  physical 
transference  and  superinduction. 

Subsequently  to  Bacon  a  belief  has  generally  existed,  and 
now  to  a  great  extent  exists*,  in  what  are  called  secondary 
causes,  or  consequential  steps,  wherein  one  phenomenon  is 
supposed  necessarily  to  hang  on  another,  until  at  last  we  ar- 
rive at  an  essential  cause,  subject  immediately  to  the  First 
Cause.  This  notion  is  generally  prevalent  both  on  the  Con- 
tinent and  in  this  country:  nothing  is  more  familiar  than  the 
expression  '  study  the  effects  in  order  to  arrive  at  the  causes/ 

Instead  of  regarding  the  proper  object  of  physical  science 
as  a  search  after  essential  causes,  I  believe  it  ought  to  be,  and 
must  be,  a  search  after  facts  and  relations — that  although  the 
word  Cause  may  be  used  in  a  secondary  and  concrete  sense, 
as  meaning  antecedent  forces,  yet  in  an  abstract  sense  it  is  to- 
tally inapplicable  ;  we  cannot  predicate  of  any  physical  agency 
that  it  is  abstractedly  the  cause  of  another  ;  and  if,  for  the  sake 
of  convenience,  the  language  of  secondary  causation  be  per- 
missible, it  should  be  only  with  reference  to  the  special  phe- 
nomena referred  to,  as  it  can  never  be  generalised. 

The  misuse,  or  rather  varied  use,  of  the  term  Cause,  has 


16  COERELATION   OF   PHYSICAL   FOKCE8. 

been  a  source  of  great  confusion  in  physical  theories,  and 
philosophers  are  even  now  by  no  means  agreed  as  to  their 
conception  of  causation.  The  most  generally  received  view 
of  causation,  that  of  Hume,  refers  it  to  invariable  antecedence 
— i.  e.,  we  call  that  a  cause  which  invariably  precedes,  that 
an  effect  which  invariably  succeeds.  Many  instances  of  in- 
variable sequence  might  however  be  selected,  which  do  not 
present  the  relation  of  cause  and  effect :  thus  as  Reed  observes, 
and  Brown  does  not  satisfactorily  answer,  day  invariably 
precedes  night  and  yet  day  is  not  the  cause  of  night.  The  seed, 
again,  precedes  the  plant,  but  is  not  the  cause  of  it ;  so  that  when 
we  study  physical  phenomena  it  becomes  difficult  to  separate  the 
idea  of  causation  from  that  of  force,  and  these  have  been  regarded 
as  identical  by  some  philosophers.  To  take  an  example  which 
will  contrast  these  two  views  :  if  a  floodgate  be  raised,  the  water 
flows  out ;  in  ordinary  parlance,  the  water  is  said  to  flow  be- 
cause the  floodgate  is  raised :  the  sequence  is  invariable ;  no 
floodgate,  properly  so  called,  can  be  raised  without  the  water 
flowing  out,  and  yet  in  another,  and  perhaps  more  strict,  sense, 
it-  is  the  gravitation  of  the  water  which  causes  it  to  flow.  But 
though  we  may  truly  say  that,  in  this  instance,  gravitation 
causes  the  water  to  flow,  we  cannot  in  truth  abstract  the  pro- 
position, and  say,  generally,  that  gravitation  is  the  cause  of 
water  flowing,  as  water  may  flow  from  other  causes,  gaseous 
elasticity,  for  instance,  which  will  cause  water  to  flow  from  a 
receiver  full  of  air  into  one  that  is  exhausted  ;  gravitation  may 
also,  under  certain  circumstances,  arrest  instead  of  cause  the 
flow  of  water. 

Upon  neither  view,  however,  can  we  get  at  anything  like 
abstract  causation.  If  we  regard  causation  as  invariable  se- 
quence, we  can  find  no  case  in  which  a  given  antecedent  is 
the  only  antecedent  to  a  given  sequent :  thus  if  water  could 
flow  from  no  other  cause  than  the  withdrawal  of  a  floodgate, 
we  might  say  abstractedly  that  this  was  the  cause  of  water 
flowing.  If,  again,  adopting  the  view  which  looks  to  causa- 


INTRODUCTORY  REMARKS.  17 

tion  as  a  force,  we  could  say  that  water  could  be  caused  to 
flow  only  by  gravitation,  we  might  say  abstractedly  that  grav- 
itation was  the  cause  of  water  flowing — but  this  we  cannot 
say ;  and  if  we  seek  and  examine  any  other  example,  we 
shall  find  that  causation  is  only  predicable  of  it  in  the  partic- 
ular case,  and  cannot  be  supported  as  an  abstract  proposition  ; 
yet  this  is  constantly  attempted.  Nevertheless,  in  each  par- 
ticular case  where  we  speak  of  Cause,  we  habitually  refer  to 
some  antecedent  power  or  force  :  we  never  see  motion  or  any 
change  in  matter  take  effect  without  regarding  it  as  produced 
by  some  previous  change  ;*  and  when  we  cannot  trace  it  to  its 
antecedent,  we  mentally  refer  it  to  one  ;  but  whether  this  hab- 
it be  philosophically  correct  is  by  no  means  clear.  In  other 
words,  it  seems  questionable,  not  only  whether  cause  and  ef- 
fect are  convertible  terms  with  antecedence  and  sequence,  but 
whether  in  fact  cause  does  precede  effect,  whether  force  does 
precede  the  change  in  matter  of  which  it  is  said  to  be  the 
cause. 

The  actual  priority  of  cause  to  effect  has  been  doubted, 
and  their  simultaneity  argued  with  much  ability.  As  an  in- 
stance of  this  argument  it  may  be  said,  the  attraction  which 
causes  iron  to  approach  the  magnet  is  simultaneous  with  and 
ever  accompanies  the  movement  of  the  iron  ;  the  movement 
is  evidence  of  the  co-existing  cause  or  force,  but  there  is  no 
evidence  of  any  interval  in  time  between  the  one  and  the  oth- 
er. On  this  view  time  would  cease  to  be  a  necessary  element 
in  causation  ;  the  idea  of  cause,  except  perhaps  as  referred  to 
a  primeval  creation,  would  cease  to  exist ;  and  the  same  ar- 
guments which  apply  to  the  simultaneity  of  cause  with  effect 
would  apply  to  the  simultaneity  of  Force  with  Motion.  We 
could  not,  however,  even  if  we  adopted  this  view,  dispense 
with  the  element  of  time  in  the  sequence  of  phenomena ;  the 
effect  being  thus  regarded  as  ever  accompanied  simultaneous- 
ly by  its  appropriate  cause,  we  should  still  refer  it  to  some  an- 
tecedent effect ;  and  our  reasoning  as  applied  to  the  succes- 
sive production  of  all  natural  changes  would  be  the  same. 


18  CORRELATION   OF   PHYSICAL   FORCES. 

Habit  and  the  identification  of  thoughts  with  phenomena 
so  compel  the  use  of  recognised  terms,  that  we  cannot  avoid 
using  the  word  cause  even  in  the  sense  to  which  objection  is 
taken ;  and  if  we  struck  it  out  of  our  vocabulary,  our  lan- 
guage, in  speaking  of  successive  changes,  would  be  unintelli- 
gible to  the  present  generation.  The  common  error,  if  I  am 
right  in  supposing  it  to  be  such,  consists  in  the  abstraction  of 
cause,  and  in  supposing  in  each  case  a  general  secondary 
cause — a  something  which  is  not  the  first  cause,  but  which,  if 
we  examine  it  carefully,  must  have  all  the  attributes  of  a  first 
cause,  and  an  existence  independent  of,  and  dominant  over, 
matter. 

The  relations  of  electricity  and  magnetism  afford  us  a 
very  instructive  example  of  the  belief  in  secondary  causa- 
tion. Subsequent  to  the  discovery  by  Oersted  of  electro-mag- 
netism, and  prior  to  that  by  Faraday  of  magneto-electricity, 
electricity  and  magnetism  were  believed  by  the  highest  author- 
ities to  stand  in  the  relation  of  cause  and  effect — i.  e.  elec- 
tricity was  regarded  as  the  cause,  and  magnetism  as  the  effect ; 
and  where  magnets  existed  without  any  apparent  electrical 
currents  to  cause  their  magnetism,  hypothetical  currents 
were  supposed,  for  the  purpose  of  carrying  out  the  caus- 
ative view ;  but  magnetism  may  now  be  said  with  equal 
truth  to  be  the  cause  of  electricity,  and  electrical  currents 
may  be  referred  to  hypothetical  magnetic  lines  :  if  therefore 
electricity  cause  magnetism,  and  magnetism  cause  electricity, 
why  then  electricity  causes  electricity,  which  becomes,  so  to 
speak,  a  reductio  ad  absurdum  of  the  doctrine. 

To  take  another  instance,  which  may  render  these  posi- 
tions more  intelligible.  By  heating  bars  of  bismuth  and  anti- 
mony in  contact,  a  current  of  electricity  is  produced ;  and  if 
their  extremities  be  united  by  a  fine  wire,  the  wire  is  heated. 
Now  here  the  electricity  in  the  metals  is  said  to  be  caused 
by  heat,  and  the  heat  in  the  wire  to  be  caused  by  electricity, 
and  in  a  concrete  sense  this  is  true ;  but  can  we  thence  say 


INTRODUCTORY   REMARKS.  19 

abstractedly  that  heat  is  the  cause  of  electricity,  or  that  elec- 
tricity is  the  cause  of  heat  ?  Certainly  not ;  for  if  either  be 
true,  both  must  be  so,  and  the  effect  then  becomes  the  cause 
of  the  cause,  or,  in  other  words,  a  thing  causes  itself.  Any 
other  proposition  on  this  subject  will  be  found  to  involve  sim- 
ilar difficulties,  until,  at  length,  the  mind  will  become  con- 
vinced that  abstract  secondary  causation  does  not  exist,  and 
that  a  search  after  essential  causes  is  vain. 

The  position  which  I  seek  to  establish  in  this  Essay  is, 
that  the  various  affections  of  matter  which  constitute  the 
main  objects  of  experimental  physics,  viz.,  heat,  light,  elec- 
tricity, magnetism,  chemical  affinity,  and  motion,  are  all  cor- 
relative, or  have  a  reciprocal  dependence  ;  that  neither,  taken 
abstractedly,  can  be  said  to  be  the  essential  cause  of  the  oth- 
ers, but  that  either  may  produce  or  be  convertible  into,  any 
of  the  others  :  thus  heat  may  mediately  or  immediately  produce 
electricity,  electricity  may  produce  heat ;  and  so  of  the  rest, 
each  merging  itself  as  the  force  it  produces  becomes  devel- 
oped :  and  that  the  same  must  hold  good  of  other  forces,  it  be- 
ing an  irresistible  inference  from  observed  phenomena  that  a 
force  cannot  originate  otherwise  than  by  devolution  from  some 
pre-existing  force  or  forces. 

The  term  force,  although  used  in  very  different  senses  by 
different  authors,  in  its  limited  sense  may  be  defined  as  that 
which  produces  or  resists  motion.  Although  strongly  inclined  to 
believe  that  the  other  affections  of  matter,  which  I  have  above 
named,  are,  and  will  ultimately  be  resolved  into,  modes  of 
motion,  many  arguments  for  which  will  be  given  in  subse- 
quent parts  of  this  Essay,  it  would  be  going  too  far,  at  pre- 
sent, to  assume  their  identity  with  it ;  I  therefore  use  the  term 
force  in  reference  to  them,  as  meaning  that  active  principle 
inseparable  from  matter  which  is  supposed  to  induce  its  vari- 
ous changes. 

The  word  force  and  the  idea  it  aims  at  expressing  might 
be  objected  to  by  the  purely  physical  philosopher  on  similar 


20  CORRELATION   OF   PHYSICAL   FORCES. 

grounds  to  those  which  apply  to  the  word  cause,  as  it  repre- 
sents a  subtle  mental  conception,  and  not  a  sensuous  percep- 
tion or  phenomenon.  The  objection  would  take  something 
of  this  form.  If  the  string  of  a  bent  bow  be  cut,  the  bow 
will  straighten  itself ;  we  thence  say  there  is  an  elastic  force 
in  the  bow  which  straightens  it ;  but  if  we  applied  our  expres- 
sions to  this  experiment  alone,  the  use  of  the  term  force 
would  be  superfluous,  and  would  not  add  to  our  knowledge 
on  the  subject.  All  the  information  which  our  minds  could 
get  would  be  as  sufficiently  obtained  from  the  expression, 
when  the  string  is  cut,  the  bow  becomes  straight,  as  from  the 
expression,  the  bow  becomes  straight  by  its  elastic  force. 
Do  we  know  more  of  the  phenomena,  viewed  without  refer- 
ence to  other  phenomena,  by  saying  it  is  produced  by  force? 
Certainly  not.  All  we  know  or  see  is  the  effect ;  we  do  not 
see  force — we  see  motion  or  moving  matter. 

If  now  we  take  a  piece  of  caoutchouc  and  stretch  it,  when 
released  it  returns  to  its  original  length.  Here,  though  the 
subject-matter  is  very  different,  we  see  some  analogy  in  the 
effect  or  phenomenon  to  that  of  the  strung  bow.  If  again 
we  suspend  an  apple  by  a  string,  cut  the  string,  the  apple  falls. 
Here,  though  it  is  less  striking,  there  is  still  an  analogy  to 
the  strung  bow  and  the  caoutchouc. 

Now  when  the  word  force  is  employed  as  •comprehending 
these  three  different  phenomena  we  find  some  use  in  the  term, 
not  by  its  explaining  or  rendering  more  intelligible  the  modus 
agendi  of  matter,  but  as  conveying  to  the  mind  something 
which  is  alike  in  the  three  phenomena,  however  distinct  they 
may  be  in  other  respects  :  the  word  becomes  an  abstract  or 
generalised  expression,  and  regarded  in  this  light  is  of  high 
utility.  Although  I  have  given  only  three  examples,  it  is 
obvious  that  the  term  would  equally  apply  to  300  or  3,000  ex- 
amples. 

But  it  will  be  said,  the  term  force  is  used  not  as  express- 
ing the  effect,  but  as  that  which  produces  the  effect.  This  is 


INTRODUCTORY   REMARKS.  21 

true,  and  in  this  its  ordinary  sense  I  shall  use  it  in  these  pages. 
But  though  the  term  has  a  potential  meaning,  to  depart  from 
which  would  render  language  unintelligible,  we  must  guard 
against  supposing  that  we  know  essentially  more  of  the  phe- 
nomena by  saying  they  are  produced  by  something,  which 
something  is  only  a  word  derived  from  the  constancy  and 
similarity  of  the  phenomena  we  seek  to  explain  by  it.  The 
relations  of  the  phenomena  to  which  the  terms  force  or  forces 
are  applied  give  us  real  knowledge  ;  these  relations  may  be 
called  relations  of  forces  ;  our  knowledge  of  them  is  not  there- 
by lessened,  and  the  convenience  of  expression  is  greatly  in- 
creased, but  the  separate  phenomena  are  not  more  intimately 
known  ;  no  further  insight  into  why  the  apple  falls  is  acquired 
by  saying  it  is  forced  to  fall,  or  it  falls  by  the  force  of  gravita- 
tion ;  by  the  latter  expression  we  are  enabled  to  relate  it 
most  usefully  to  other  phenomena,  but  we  still  know  no  more 
of  the  particular  phenomenon  than  that  under  certain  circum- 
stances the  apple  does  fall. 

In  the  above  illustrations,  force  has  been  treated  as  the 
producer  of  motion,  in  which  case  the  evidence  of  the  force  is 
the  motion  produced  ;  thus  we  estimate  the  force  used  to  pro- 
ject a  cannon  ball  in  terms  of  the  mass  of  matter,  and  the 
velocity  with  which  it  is  projected.  The  evidence  of  force 
when  the  term  is  applied  to  resistance  to  motion  is  of  a  some- 
what different  character  ;  the  matter  resisting  is  molecularly 
affected,  and  has  its  structure  more  or  less  changed ;  thus  a 
strip  of  caoutchouc  to  which  a  weight  is  suspended  is  elonga- 
ted, and  its  molecules  are  displaced  as  compared  with  their 
position  when  unaffected  by  the  gravitating  force.  So  a  piece 
of  glass  bent  by  an  appended  weight  has  its  whole  structure 
changed ;  this  internal  change  is  made  evident  by  transmit- 
ting through  it  a  beam  of  polarised  light :  a  relation  thus 
becomes  established  between  the  molecular  state  of  bodies 
and  the  external  forces  or  motion  of  masses.  Every  particle 
of  the  caoutchouc  or  glass  must  be  acting  and  contributing  to 


2Sa  CORRELATION   OF   PHYSICAL   FORCES. 

resist  or  arrest  the  motion  of  the  mass  of  matter  appended 
to  it. 

It  is  difficult,  in  such  cases,  not  to  recognise  a  reality  in 
force.  We  need  some  word  to  express  this  state  of  tension  ; 
we  know  that  it  produces  an  effect,  though  the  effect  be  nega- 
tive in  character  :  although  in  this  effort  of  inanimate  matter 
we  can  no  more  trace  the  mode  of  action  to  its  ultimate  ele- 
ments than  we  can  follow  out  the  connection  of  our  own 
muscles  with  the  volition  which  calls  them  into  action,  we 
are  experimentally  convinced  that  matter  changes  its  state 
by  the  agency  of  other  matter,  and  this  agency  we  call 
force. 

In  placing  the  weight  on  the  glass,  we  have  moved  the 
former  to  an  extent  equivalent  to  that  which  it  would  again 
describe  if  the  resistance  were  removed,  and  this  motion  of 
the  mass  becomes  an  exponent  or  measure  of  the  force  exert- 
ed on  the  glass ;  while  this  is  in  the  state  of  tension,  the 
force  is  ever  existing,  capable  of  reproducing  the  original 
motion,  and  while  in  a  state  of  abeyance  as  to  actual  motion, 
it  is  really  acting  on  the  glass.  The  motion  is  suspended, 
but  the  force  is  not  annihilated. 

But  it  may  be  objected,  if  tension  or  static  force  be  thus 
motion  in  abeyance,  there  is  at  all  times  a  large  amount  of 
dynamical  action  subtracted  from  the  universe.  Every  stone 
upon  a  hill,  every  spring  that  is  bent,  and  has  required  force 
to  upraise  or  bend  it,  has  for  a  time,  and  possibly  for  ever, 
withdrawn  this  force,  and  annihilated  it.  Not  so ;  what 
takes  place  when  we  raise  a  weight  and  leave  it  at  the  point 
to  which  it  has  been  elevated  ?  we  have  changed  the  centre 
of  gravity  of  the  earth,  and  consequently  the  earth's  position 
with  reference  to  the  sun,  planets,  and  stars ;  the  effort  we 
have  made  pervades  and  shakes  the  universe ;  nor  can  we 
present  to  the  mind  any  exercise  of  force,  which  is  thus  not 
permanent  in  its  dynamical  effects.  If,  instead  of  one  weight 
being  raised,  we  raise  two  weights,  each  placed  at  a  point 


INTRODUCTOEY   REMARKS.  23 

diametrically  opposite  the  other,  it  would  be  said,  here  you 
have  compensation,  a  balance,  no  change  in  the  centre  of 
gravity  of  the  earth  ;  but  we  have  increased  the  mean  diame- 
ter of  the  earth,  and  a  perturbation  of  our  planet,  and  of  all 
other  celestial  bodies  necessarily  ensues. 

The  force  may  be  said  to  be  in  abeyance  with  reference 
to  the  effect  it  would  have  produced,  if  not  arrested,  or 
placed  in  a  state  of  tension ;  but  in  the  act  of  imposing  this 
state,  the  relations  of  equilibrium  with  other  bodies  have 
been  changed,  and  these  move  in  their  turn,  so  that  motion 
of  the  same  amount  would  seem  to  be  ever  affecting  matter 
conceived  in  its  totality. 

Press  the  hands  violently  together ;  the  first  notion  may 
be  that  this  is  power  locked  up,  and  that  no  change  ensues. 
Not  so  ;  the  blood  courses  more  quickly,  respiration  is  accele- 
rated, changes  which  we  may  not  be  able  to  trace,  take  place 
in  the  muscles  and  nerves,  transpiration  is  increased ;  we 
have  given  off  force  in  various  ways,  and  must,  if  the  effort 
be  prolonged,  replenish  our  sources  of  power,  by  fresh  chemi- 
cal action  in  the  stomach. 

In  books  which  treat  of  statics  and  dynamics,  it  is  com- 
mon and  perhaps  necessary  to  isolate  the  subjects  of  consid- 
eration ;  to  suppose,  for  instance,  two  bodies  gravitating,  and 
to  ignore  the  rest  of  the  universe.  But  no  such  isolation  ex- 
ists in  reality,  nor  could  we  predict  the  result  if  it  did  exist. 
Would  two  bodies  gravitate  towards  each  other  in  empty 
space,  if  space  can  be  empty?  the  notion  that  they  would  is 
founded  on  the  theory  of  attraction,  which  Newton  himself 
repudiated,  further  than  as  a  convenient  means  of  regard- 
ing the  subject.  For  purposes  of  instruction  or  argument  it 
may  be  convenient  to  assume  isolated  matter :  many  con- 
clusions so  arrived  at  may  be  true,  but  many  will  be 
erroneous. 

If,  in  producing  effects  of  tension  or  of  static  force,  the 
effort  made  pervades  the  universe,  it  may  be  said,  when  the 


24  CORRELATION   OF  PHYSICAL  FORCES. 

bent  spring  is  freed,  when  the  raised  weight  falls,  a  converse 
series  of  motions  must  be  effected,  and  this  theory  would  lead 
to  a  mere  reciprocation,  which  would  be  equally  unproduc- 
tive of  permanent  change  with  the  annihilation  of  force.  If 
raising  the  weight  has  changed  the  centre  of  gravity  of  the 
earth,  and  thence  of  the  universe,  the  fall  of  the  weight,  it 
will  be  said,  restores  the  original  centre  of  gravity,  and  every- 
thing comes  back  to  its  original  status.  In  this  argument  we 
again,  in  thought,  isolate  our  experiment ;  we  neglect  sur- 
rounding circumstances.  Between  the  time  of  the  raising 
and  falling  of  the  weight,  be  the  interval  never  so  small, 
nay,  more,  during  the  rising  and  during  the  fall,  the  earth 
has  been  going  on  revolving  round  its  axis  and  round  the 
sun,  to  say  nothing  of  other  changes,  such  as  temperature, 
cosmical  magnetism,  &c.,  which  we  may  call  accidental,  but 
which,  if  we  knew  all,  would  probably  be  found  to  be  as 
necessary  and  as  reducible  to  law  as  the  motion  of  the  earth. 
A  change  having  taken  place,  the  fall  of  the  weight  does 
not  bring  back  the  status  quo,  but  other  changes  supervene, 
and  so  on.  Nothing  repeats  jtself,  because  nothing  can 
be  placed  again  in  the  same  condition:  the  past  is  irre- 
vocable. 


II.— MOTION. 

MOTION — which  has  been  taken  as  the  main  exponent 
of  force  in  the  above  examples — is  the  most  obvious? 
the  most  distinctly  conceived  of  all  the  affections  of  matter. 
Visible  motion,  or  relative  change  of  position  in  space,  is  a 
phenomenon  so  obvious  to  simple  apprehension,  that  to  at- 
tempt to  define  it  would  be  to  render  it  more  obscure  ;  but 
with  motion,  as  with  all  physical  appearances,  there  are  cer- 
tain vanishing  gradations  or  undefined  limits,  at  which  the 
obvious  mode  of  action  fades  away  ;  to  detect  the  continu- 
ing existence  of  the  phenomena  we  are  obliged  to  have  re- 
course to  other  than  ordinary  methods  of  investigation,  and 
we  frequently  apply  other  and  different  names  to  the  effects 
so  recognised. 

Thus  sound  is  motion ;  and  although  in  the  earlier  pe- 
riods of  philosophy  the  identity  of  sound  and  motion  was  not 
traced  out,  and  they  were  considered  distinct  affections  of 
matter — indeed,  at  the  close  of  the  last  century  a  theory  was 
advanced  that  sound  was  transmitted  by  the  vibrations  of  an 
ether — we  now  so  readily  resolve  sound  into  motion,  that  to 
those  who  are  familiar  with  acoustics,  the  phenomena  ef 
sound  immediately  present  to  the  mind  the  idea  of  motion, 
i.  e.  motion  of  ordinary  matter. 

Again,  with  regard  to  light :  no  doubt  now  exists  that 
light  moves  or  is  accompanied  by  motion.     Here  the  phe- 
2 


26  CORRELATION   OF   PHYSICAL   FORCES. 

nomena  of  motion  are  not  made  evident  by  the  ordinary  sen- 
suous perception,  as  for  instance  the  motion  of  a  visibly  mov- 
ing projectile  would  be,  but  by  an  inverse  deduction  from 
known  relations  of  motion  to  time  and  space  :  as  all  observa- 
tion teaches  us  that  bodies  in  moving  from  one  point  in  space 
to  another  occupy  time,  we  conclude  that,  wherever  a  con- 
tinuing phenomenon  is  rendered  evident  in  two  different 
points  of  space  at  different  times,  there  is  motion,  though  we 
cannot  see  the  progression.  A  similar  deduction  convinces 
us  of  the  motion  of  electricity. 

As  we  in  common  parlance  speak  of  sound  moving, 
although  sound  is  motion,  it  requires  no  great  stretch  of 
imagination  to  conceive  light  and  electricity  as  motions,  and 
not  as  things  moving.  If  one  end  of  a  long  bar  of  metal  be 
struck,  a  sound  is  soon  perceptible  at  the  other  end.  This 
we  now  know  to  be  a  vibration  of  the  bar ;  sound  is  but  a  word 
expressive  of  the  mode  of  motion  impressed  on  the  bar ;  so 
one  end  of  a  column  of  air  or  glass  subjected  to  a  luminous  im- 
pulse gives  a  perceptible  effect  of  light  at  the  other  end :  this 
can  equally  be  conceived  to  be  a  vibration  or  transmitted 
motion  of  particles  in  the  transparent  column :  this  question 
will,  however,  be  further  discussed  hereafter  ;  for  the  present 
we  will  confine  ourselves  to  motion  within  the  limits  to  which 
the  term  is  usually  restricted. 

With  the  perceptible  phenomena  of  motion  the  mental 
conception  has  been  invariably  associated  to  which  I  have 
before  alluded,  and  to  which  the  term  force  is  given — 
the  which  conception,  when  we  analyse  it,  refers  us  to 
some  antecedent  motion.  If  we  except  the  production  of 
motion  by  heat,  light,  &c.,  which  will  be  considered  in  the 
sequel,  when  we  see  a  body  moving  we  look  to  motion  hav- 
ing been  communicated  to  it  by  matter  which  has  previously 
moved. 

Of  absolute  rest  Nature  gives  us  no  evidence  :  all  matter, 
as  far  as  we  can  ascertain,  is  ever  in  movement,  not  merely 


MOTION.  21 

in  masses,  as  with  the  planetary  spheres,  but  also  mole- 
cularly,  or  throughout  its  most  intimate  structure  :  thus  every 
alteration  of  temperature  produces  a  molecular  change 
throughout  the  whole  substance  heated  or  cooled ;  slow 
chemical  or  electrical  actions,  actions  of  light  or  invisible 
radiant  forces,  are  always  at  play,  so  that  as  a  fact  we  can- 
not predicate  of  any  portion  of  matter  that  it  is  absolutely  at 
rest.  Supposing,  however,  that  motion  is  not  an  indispensa- 
ble function  of  matter,  but  that  matter  can  be  at  rest,  matter 
at  rest  would  never  of  itself  cease  to  be  at  rest ;  it  would  not 
move  unless  impelled  to  such  motion  by  some  other  moving 
body,  or  body  which  has  moved.  This  proposition  applies 
not  merely  to  impulsive  motion,  as  when  a  ball  at  rest  is 
struck  by  a  moving  body,  or  pressed  by  a  spring  which  has 
previously  been  moved,  but  to  motion  caused  by  attractions 
such  as  magnetism  or  gravitation.  Suppose  a  piece  of  iron 
at  rest  in  contact  with  a  magnet  at  rest ;  if  it  be  desired  to 
move  the  iron  by  the  attraction  of  the  magnet,  the  magnet  or 
the  iron  must  first  be  moved  ;  so  before  a  body  falls  it  must 
first  be  raised.  A  body  at  rest  would  therefore  continue  so 
for  ever,  and  a  body  once  in  motion  would  continue  so  for 
ever,  in  the  same  direction  and  with  the  same  velocity,  un- 
less impeded  by  some  other  body,  or  affected  by  some  other 
force  than  that  which  originally  impelled  it.  These  propo- 
sitions may  seem  somewhat  arbitrary,  and  it  has  been  doubted 
whether  they  are  necessary  truths  ;  they  have  for  a  long  time 
been  received  as  axioms,  and  there  can  at  all  events  be  no 
harm  in  accepting  them  as  postulates.  It  is  however  very 
generally  believed  that  if  the  visible  or  palpable  motion  of 
one  body  be  arrested  by  impact  on  another  body,  the  mo- 
tion ceases,  and  the  force  which  produced  it  is  annihi- 
lated. 

Now  the  view  which  I  venture  to  submit  is,  that  force 
cannot  be  annihilated,  but  is  merely  subdivided  or  altered  in 
direction  or  character.  First,  as  to  direction.  Wave  your 


28  CORRELATION  OF   PHYSICAL   FORCES. 

hand :  the  motion,  which  has  apparently  ceased,  is  taken  up 
by  the  air,  from  the  air  by  the  walls  of  the  room,  &c.,  and 
so  by  direct  and  reacting  waves,  continually  comminuted,  but 
never  destroyed.  It  is  true  that,  at  a  certain  point,  we  lose 
all  means  of  detecting  the  motion,  from  its  minute  subdivi- 
sion, which  defies  our  most  delicate  means  of  appreciation, 
but  we  can  indefinitely  extend  our  power  of  detecting  it  ac- 
cording as  we  confine  its  direction,  or  increase  the  delicacy 
of  our  examination.  Thus,  if  the  hand  be  moved  in  uncon- 
fined  air,  the  motion  of  the  air  would  not  be  sensible  to  a  per- 
son at  a  few  feet  distance  ;  but  if  a  piston  of  the  same  extent 
of  surface  as  the  hand  be  moved  with  the  same  rapidity  in  a 
tube,  the  blast  of  air  may  be  distinctly  felt  at  several  yards 
distance.  There  is  no  greater  absolute  amount  of  motion  in 
the  air  in  the  second  than  in  the  first  case,  but  its  direction 
is  restrained,  so  to  make  the  means  of  detection  more  facile. 
By  carrying  on  this  restraint,  as  in  the  air-gun,  we  get  a 
power  of  detecting  the  motion,  and  of  moving  other  bodies  at 
far  greater  distances.  The  puff  of  air  which  would  in  the 
air-gun  project  a  bullet  a  quarter  of  a  mile,  if  allowed  to  es- 
cape without  its  direction  being  restrained,  as  by  the  bursting 
of  a  bladder,  would  not  be  perceptible  at  a  yard  distance, 
though  the  same  absolute  amount  of  motion  be  impressed  on 
the  surrounding  air. 

It  may,  however,  be  asked,  what  becomes  of  force  when 
motion  is  arrested  or  impeded  by  the  counter-motion  of  another 
body  ?  This  is  generally  believed  to  produce  rest,  or  entire 
destruction  of  motion,  and  consequent  annihilation  of  force : 
so  indeed  it  may,  as  regards  the  motion  of  the  masses,  but  a 
new  force,  or  new  character  of  force,  now  ensues,  the  expo- 
nent of  which,  instead  of  visible  motion,  is  heat.  I  venture 
to  regard  the  heat  which  results  from  friction  or  percussion 
as  a  continuation  of  the  force  which  was  previously  associa- 
ted with  the  moving  body,  and  which,  when  this  impinges  on 


MOTION.  29 

another  body,  ceasing  to  exist  as  gross,  palpable  motion,  con- 
tinues to  exist  as  heat. 

Thus,  let  two  bodies,  A  and  B,  be  supposed  to  move  in 
opposite  directions  (putting  for  the  moment  out  of  question 
all  resistance,  such  as  that  of  the  air,  &c.),  if  they  pass  each 
other  without  contact  each  will  move  on  for  ever  in  its  re- 
spective direction  with  the  same  velocity,  but  if  they  touch 
each  other  the  velocity  of  the  movement  of  each  is  reduced, 
and  each  becomes  heated :  if  this  contact  be  slight,  or  such  as 
to  occasion  but  a  slight  diminution  of  their  velocity,  as  when 
the  surfaces  of  the  bodies  are  oiled,  then  the  heat  is  slight ; 
but  if  the  contact  be  such  as  to  occasion  a  great  diminution 
of  motion,  as  in  percussion,  or  as  when  the  surfaces  are 
roughened,  then  the  heat  is  great,  so  that  in  all  cases  the  re- 
sulting heat  is  proportionate  to  the  diminished  velocity. 
Where,  instead  of  resisting  and  consequently  impeding  the 
motion  of  the  body  A,  the  body  B  gives  way,  or  itself  takes 
up  the  motion  originally  communicated  to  A,  then  we  have 
less  heat  in  proportion  to  the  motion  of  the  body  B,  for  here 
the  operation  of  the  force  continues  in  the  form  of  palpable 
motion :  thus  the  heat  resulting  from  friction  in  the  axle  of  a 
wheel  is  lessened  by  surrounding  it  by  rollers  ;  these  take  up 
the  primary  motion  of  the  axle,  and  the  less,  by  this  means, 
the  initial  motion  is  impeded,  the  less  is  the  resulting  heat. 
Again,  if  a  body  move  in  a  fluid,  although  some  Jjeat  is  pro- 
duced, the  heat  is  apparently  trifling,  because  the  particles  of 
the  fluid  themselves  move,  and  continue  the  motion  originally 
communicated  to  the  moving  body :  for  every  portion  of  mo- 
tion communicated  to  them  this  loses  an  equivalent,  and 
where  both  lose,  then  an  equivalent  of  heat  results. 

As  the  converse  of  this  proposition,  it  should  follow  that 
the  more  rigid  the  bodies  impinging  on  each  other  the  greater 
should  be  the  amount  of  heat  developed  by  friction,  and  so 
we  find  it.  Flint,  steel,  hard  stones,  glass,  and  metals,  are 
those  bodies  which  give  the  greatest  amount  of  heat  from 


30  CORRELATION    OF   PHYSICAL   FORCES. 

friction  or  percussion ;  while  water,  oil,  &c.,  give  little  or  no 
heat,  and  from  the  ready  mobility  of  their  particles  lessen  its 
developement  when  interposed  between  rigid  moving  bodies. 
Thus,  if  we  oil  the  axles  of  wheels,  we  have  more  rapid  mo- 
tion of  the  bodies  themselves,  but  less  heat ;  if  we  increase 
the  resistance  to  motion,  as  by  roughening  the  points  of  con- 
tact, so  that  each  particle  strikes  against  and  impedes  the 
motion  of  others,  then  we  have  diminished  motion,  but  in- 
creased heat ;  or  if  the  bodies  be  smooth,  but  instead  of  slid- 
ing past  each  other  be  pressed  closely  together  and  then 
rubbed,  we  shall  in  many  cases  evolve  more  heat  than  by  the 
roughened  bodies,  as  we  get  a  greater  number  of  particles  in 
contact  and  a  greater  resistance  to  the  initial  motion.  I  can- 
not present  to  my  mind  any  case  of  heat  resulting  from  fric- 
tion which  is  not  explicable  by  this  view  :  friction,  according 
to  it,  is  simply  impeded  motion.  The  greater  the  impedi- 
ment, the  more  force  is  required  to  overcome  it,  and  the 
greater  is  the  resulting  heat ;  this  resulting  heat  being  a  con- 
tinuation of  indestructible  force,  capable,  as  we  shall  pres- 
ently see,  of  reproducing  palpable  motion,  or  motion  of  defi- 
nite masses. 

Whatever  be  the  nature  of  the  bodies,  rough  or  smooth, 
solid  or  liquid,  provided  there  be  the  same  initial  force,  and 
the  whole  motion  be  ultimately  arrested,  there  should  be  the 
same  amount  of  heat  developed,  though  where  the  motion  is 
carried  on  through  a  great  number  of  points  of  matter  we  do 
not  so  sensibly  perceive  the  resulting  heat  from  its  greater 
dissipation.  The  friction  of  fluids  produces  heat,  an  effect 
first  noticed  I  believe  by  Mayer.  The  total  heat  produced  by 
the  friction  of  fluids  should,  therefore,  it  will  be  said,  be 
equal  to  that  produced  by  the  friction  of  solids  ;  for  although 
each  particle  produces  little  heat,  the  motion  being  readily 
taken  up  by  the  neighbouring  particles,  yet  by  the  time  the 
whole  mass  has  attained  a  state  of  rest  there  has  been  the 
same  impeding  of  the  initial  motion  as  by  the  friction  of  sol- 


MOTION.  31 

ids  if  produced  by  the  same  initial  force.  If  the  heat  be 
viewed  in  the  aggregate,  and  allowance  be  made  for  the  spe 
cific  thermal  capacity  of  the  substances  employed,  it  probably 
is  the  same,  though  apparently  less  ;  the  heat  in  the  case  of 
solids  being  manifested  at  certain  defined  points,  while  in 
that  of  fluids  it  is  dissipated,  both  the  time  and  space  during 
and  through  which  the  motion  is  propagated  differ  in  the  two 
cases,  so  that  the  heat  in  the  latter  case  is  more  readily  car- 
ried off  by  surrounding  bodies. 

If  the  body  be  elastic,  and  by  its  reaction  the  motion  im- 
pressed on  it  by  the  initial  force  be  continued,  then  the  heat 
is  proportionately  less ;  and  were  a  substance  perfectly  elas- 
tic, and  no  resistance  opposed  to  it  by  the  air  or  other  mat- 
ter, then  the  movement  once  impressed  would  be  perpetual, 
and  no  heat  would  result.  A  ball  of  caoutchouc  bandied 
about  for  many  minutes  between  a  racket  and  a  wall  is  not 
perceptibly  heated,  while  a  leaden  bullet  projected  by  a  gun 
against  a  wall  is  rendered  so  hot  as  to  be  intolerable  to  the 
touch :  in  the  former  case,  the  motion  of  the  mass  is  contin- 
ued by  the  reaction  due  to  its  elasticity ;  in  the  latter,  the 
motion  of  the  mass  is  extinguished,  and  heat  ensues. 

A  pendulum  started  in  the  exhausted  receiver  of  an  air- 
pump  continues  its  oscillation  for  hours  or  even  days ;  the 
friction  at  its  point  of  suspension  and  the  resistance  of  the 
air  is  minimised,  and  the  heat  is  imperceptible,  but  these  tri- 
fling resistances  in  the  end  arrest  the  motion  of  the  mass,  the 
one  giving  it  out  as  heat,  the  other  conveying  the  force  to  the 
receiver,  and  thence  to  surrounding  bodies.  Similar  reason- 
ing may  be  applied  to  the  oscillation  of  a  coiled  spring  and 
balance  wheel. 

To  wind  up  a  clock  a  certain  amount  of  force  is  expended 
by  the  arm ;  this  force  is  given  back  by  the  descent  of  the 
weight,  the  wheels  move,  the  pendulum  is  kept  oscillating, 
heat  is  generated  at  each  point  of  friction,  and  the  surround- 
ing air  is  set  in  motion,  a  part  of  which  is  made 


32  COEKELATION   OF   PHYSICAL   FORCES. 

us  by  the  ticking  sound.  But  it  will  be  said,  if  instead  of 
allowing  the  weight  to  act  upon  the  machinery,  the  cord  by 
which  it  is  suspended  be  cut,  the  weight  drops  and  the  force 
is  at  an  end.  By  no  means,  for  in  this  case  the  house  is 
shaken  by  the  concussion,  and  thus  the  force  and  motion  are 
continued,  while  in  the  former  Case  the  weight  reaches  the 
ground  quietly,  and  no  evidence  of  force  or  motion  is  mani- 
fested by  its  impact,  the  whole  having  been  previously  dissi- 
pated. 

If  the  initial  motion,  instead  of  being  arrested  by  the  im- 
pact of  other  bodies,  as  in  friction  or  percussion,  is  impeded 
by  confinement  or  compression,  as  where  the  dilatation  of  a 
gas  is  prevented  by  mechanical  means,  heat  equally  results  : 
thus  if  a  piston  is  used  to  compress  air  in  a  closed  vessel,  the 
compressed  air  and,  from  it,  the  sides  of  the  vessel  will  be 
heated :  the  air  being  unable  to  take  up  and  carry  on  the 
original  motion  communicates  molecular  motion  or  expansion 
to  all  bodies  in  contact  with  it ;  and,  conversely,  if  we  ex- 
pand air  by  mechanical  motion,  as  by  withdrawing  the  pis- 
ton, cold  is  produced.  So  when  a  solid  has  its  particles  com- 
pressed or  brought  nearer  together,  as  when  a  bar  of  iron  is 
hammered,  heat  is  produced  beyond  that  which  is  due  to  per- 
cussion alone.  In  this  latter  case  we  cannot  very  easily  ef- 
fect the  converse  result,  or  produce  cold  by  the  mechanical 
dilatation  of  a  solid,  though  the  phenomena  of  solution, 
where  the  particles  of  a  solid  are  detached  from  each  other, 
or  drawn  more  widely  asunder,  give  us  an  approximation  to 
it :  in  the  case  of  solution  cold  is  produced. 

We  are  from  a  very  extensive  range  of  observation  and 
experiment  entitled  to  conclude  that,  with  some  curious  ex- 
ceptions to  be  presently  noticed,  whenever  a  body  is  com- 
pressed or  brought  into  smaller  dimensions  it  is  heated,  i.  e. 
it  expands  neighbouring  substances.  Whenever  it  is  dilated 
or  increased  in  volume  it  is  cooled,  or  contracts  neighbouring 
substances. 


MOTION.  33 

Mr.  Joule  has  made  a  great  number  of  experiments  for 
the  purpose  of  ascertaining  what  quantity  of  heat  is  produced 
by  a  given  mechanical  action.  His  mode  of  experimenting 
is  as  follows.  An  apparatus  formed  of  floats  or  paddles  of 
brass  or  iron  is  made  to  rotate  in  a  bath  of  water  or  mercu- 
ry. The  power  which  gives  rise  to  this  rotation  is  a  weight 
raised  like  a  clock-weight  to  a  certain  height ;  this  by  acting 
during  its  fall  on  a  spindle  and  pulley  communicates  motion 
to  the  paddle-wheel,  the  water  or  mercury  serving  as  a  fric- 
tion medium  and  calorimeter ;  and  the  heat  is  measured  by  a 
delicate  mercurial  thermometer.  The  results  of  his  experi- 
ments he  considers  prove  that  a  fall  of  772  Ibs.  through  a 
space  of  one  foot  is  able  to  raise  the  temperature  of  one 
pound  of  water  through  one  degree  of  Fahrenheit's  thermom- 
eter. Mr.  Joule's  experiments  are  of  extreme  delicacy — he 
tabulates  to  the  thousandth  part  of  a  degree  of  Fahrenheit, 
and  a  large  number  of  his  thermometric  data  are  compre- 
hended within  the  limits  of  a  single  degree.  Other  experi- 
menters have  given  very  different  numerical  results,  but  the 
general  opinion  seems  to  be  that  the  numbers  given  by  Mr. 
Joule  are  the  nearest  approximation  to  the  truth  yet  obtained. 

Hitherto  I  have  taken  no  distinction  as  to  the  physical 
character  of  the  bodies  impinging  on  each  other ;  but  Nature 
gives  us  a  remarkable  difference  in  the  character  or  mode  of 
the  force  eliminated  by  friction,  accordingly  as  the  bodies 
which  impinge  are  homogeneous  or  heterogeneous :  if  the 
former,  heat  alone  is  produced ;  if  the  latter,  electricity. 

We  find,  indeed,  instances  given  by  authors,  of  electricity 
resulting  from  the  friction  of  homogeneous  bodies  ;  but,  as  I 
stated  in  my  original  Lectures,  I  have  not  found  such  facts 
confirmed  by  my  own  experiments,  and  this  conclusion  has 
been  corroborated  by  some  experiments  of  Professor  Erman, 
communicated  to  the  meeting  of  the  British  Association  in 
the  year  1845,  in  which  he  found  that  no  electricity  resulted 
from  the  friction  of  perfectly  homogeneous  substances ;  as, 
2* 


34:  CORRELATION   OF   PHYSICAL   FORCES. 

for  instance,  the  ends  of  a  broken  bar.  Such  experiments  as 
these  will,  indeed,  be  seldom  free  from  slight  electrical  cur- 
rents, on  account  of  the  practical  difficulty  of  fulfilling  the 
condition  of  perfect  homogeneity  in  the  substances  themselves, 
their  size,  their  temperature,  &c. ;  but  the  effects  produced 
are  very  trifling  and  vary  in  direction,  and  the  resultant  effect 
is  nought.  Indeed,  it  would  be  difficult  to  conceive  the  con- 
trary. How  could  we  possibly  image  to  the  mind  or  de- 
scribe the  direction  of  a  current  from  the  same  body  to  the 
same  body,  or  give  instructions  for  a  repetition  of  the  exper- 
iment ?  It  would  be  unintelligible  to  say  that  in  rubbing  to 
and  fro  two  pieces  of  bismuth,  iron,  or  glass,  a  current  of 
electricity  circulated  from  bismuth  to  bismuth,  or  from  iron 
to  iron,  or  from  glass  to  glass  ;  for  the  question  immediately 
occurs — from  which  bismuth  to  which  does  it  circulate? 
And  should  this  question  be  answered  by  calling  one  piece 
A,  and  the  other  B,  this  would  only  apply  to  the  particular 
specimens  employed,  the  distinctive  appellation  denoting  a 
distinction  in  fact,  as  otherwise  A  could  be  substituted  for  B, 
and  the  bar  to  which  the  positive  electricity  flowed  would  in 
turn  become  the  bar  to  which  the  negative  electricity  flowed. 
We  may  say  that  it  circulates  from  rough  glass  to  smooth, 
from  cast  iron  to  wrought,  for  here  there  is  not  homogeneity. 
It  is  moreover  conceivable,  that  when  the  motion  is  contin- 
uous in  a  definite  direction,  electricity  may  result  from  the 
friction  of  homogeneous  bodies.  If  A  and  B  rub  against 
each  other,  revolving  in  opposite  directions,  concentric  cur- 
rents of  positive  and  negative  electricity  may  be  conceived 
circulating  within  the  metals,  and  be  described  by  reference 
to  the  direction  of  their  motion  ;  this  indeed  would  be  a  dif- 
ferent phenomenon  from  those  we  have  been  considering ;  but 
without  some  distinction  between  the  two  substances  in  qual- 
ity or  direction,  the  electrical  effects  are  indescribable,  if  not 
inconceivable. 

When,  however,  homogeneous  bodies  are  fractured  ot 


MOTION.  35 

even  rubbed  together,  phenomena  are  observed  to  which  the 
term  electricity  is  applied ;  a  flash  or  line  of  light  appears  at 
the  point  of  friction  which  by  some  is  called  electrical,  by 
others  phosphorescent. 

I  have  myself  observed  a  remarkable  case  of  the  kind  in 
the  caoutchouc  fabric  now  commonly  used  for  waterproof 
clothing :  if  two  folds  of  this  substance  be  allowed  to  cohere 
so  as  partly  to  unite  and  present  a  difficulty  of  separation, 
then,  on  stripping  the  one  from  the  other,  or  tearing  them 
asunder,  a  line  of  light  will  follow  the  line  of  separation. 

If  this  class  of  phenomena  be  electrical,  it  is  electricity 
determined  as  it  is  generated ;  there  is  no  dual  character  im- 
pressed on  the  matter  acting,  the  flash  is  electrical  as  a  spark 
from  the  percussion  of  flint  is  electrical,  or  as  the  slow  com- 
bustion of  phosphorus,  or  any  other  case  of  the  development 
of  heat  and  light.  It  seems  to  be  better  to  class  this  phe- 
nomenon under  the  categories  of  heat  and  light  than  under 
that  of  electricity,  the  latter  word  being  retained  for  those 
cases  where  a  dual  or  polar  character  of  force  is  manifested. 
In  experiments  which  have  been  made  by  the  friction  of  sim- 
ilar substances  where  the  one  appears  positively  and  the 
other  negatively  electrical,  there  will  be  found  some  differr 
ence  in  the  mode  of  rubbing  by  which  the  molecular  state  of 
the  bodies  is  in  all  probability  changed,  making  one  a  dissim- 
ilar substance  from  the  other ;  thus  it  is  said  by  Bergmann, 
that  when  two  pieces  of  glass  are  rubbed  so  that  all  the  parts 
of  one  pass  over  one  part  of  the  other,  the  former  is  positive 
and  the  latter  negative.  It  is  obvious  that  in  this  case  the 
rubbing  in  one  is  confined  to  a  line,  and  that  must  be  more 
altered  in  molecular  structure  at  the  line  of  friction  than  the 
one  where  the  friction  is  spread  over  the  whole  surface :  so 
if  a  ribbon  be  drawn  transversely  over  another  ribbon,  the 
substances  are  not,  qua  the  rubbing  action,  identical;  so 
again,  in  the  rupture  of  crystals,  we  are  dealing  with  sub- 
stances having  a  polar  arrangement  of  particles — the  surfaces 


00  CORRELATION   OF  PHYSICAL   FORCES. 

of  the  fragments  cannot  be  assumed  to  be  molecularly  identi- 
cal^ 

The  developement  of  electricity  by  the  common  electrical 
machine  arises,  as  far  as  I  can  understand  it,  from  the  sepa- 
ration or  rupture  of  contiguity  between  dissimilar  bodies ;  a 
metallic  surface,  the  amalgam  of  the  cushion,  is  in  contact 
with  glass  ;  these  two  bodies  act  upon  each  other  by  the  force 
of  cohesion ;  and  when,  by  an  external  mechanical  force, 
this  is  ruptured,  as  it  is  at  each  moment  of  the  motion  of  the 
glass  plate  or  cylinder,  electricity  is  developed  in  each  ;  were 
they  similar  bodies,  heat  only  would  be  developed. 

According  to  the  experiments  of  Mr.  Sullivan  electricity 
may  be  produced  by  vibration  alone  if  the  substance  vibra- 
ting be  composed  either  of  dissimilar  metals,  as  a  wire  partly 
of  iron  and  partly  of  brass  caused  to  emit  a  musical  sound ; 
or  of  the  same  metal,  if  its  parts  be  not  homogeneous,  as  a 
piece  of  iron,  one  portion  of  which  is  hard  and  crystallised 
and  the  other  soft  and  fibrous ;  the  current  resulting  appears 
to  be  due  to  the  vibration,  and  not  to  heat  engendered,  as  it 
ceases  immediately  with  the  vibration. 

We  may  say,  then,  that  in  our  present  state  of  knowledge, 
where  the  mutually  impinging  bodies  are  homogeneous,  heat 
and  not  electricity  is  the  result  of  friction  and  percussion ; 
where  the  bodies  impinging  are  heterogeneous,  we  may  safely 
state  that  electricity  is  always  produced  by  friction  or  percus- 
sion, although  heat  in  a  greater  or  less  degree  accompaDies 
it ;  but  when  we  come  to  the  question  of  ratio  in  which  fric- 
tional  electricity  is  produced,  as  determined  by  the  different 
characters  of  the  substances  employed,  we  find  very  complex 
results.  Bodies  may  differ  in  so  many  particulars  which  in- 
fluence more  or  less  the  developement  of  electricity,  such  as 
their  chemical  constitution,  the  state  of  their  surfaces,  their 
state  of  aggregation,  their  transparency  or  opacity,  their 
power  of  conducting  electricity,  &c.,  that  the  normce  of  their 
action  are  very  difficult  of  attainment.  As  a  general  rule,  il 


MOTION.  37 

may  be  said  that  the  developement  of  electricity  is  greater 
when  the  substances  employed  are  broadly  distinct  in  their 
physical  and  chemical  qualities,  and  more  particularly  in  their 
conducting  powers  ;  but  up  to  the  present  time  the  laws  gov- 
erning such  developement  have  not  been  even  approximately 
determined. 

I  have  said,  in  reference  to  the  various  forces  or  affections 
of  matter,  that  either  of  them  may,  mediately  or  immediately, 
produce  the  others  ;  and  this  is  all  I  can  venture  to  predicate 
of  them  in  the  present  state  of  science  ;  but  after  much  con- 
sideration I  incline  strongly  to  the  opinion  that  science  is  rap- 
idly progressing  towards  the  establishment  of  immediate  or 
direct  relations  between  all  these  forces.  Where  at  present 
no  immediate  relation  is  established  between  any  of  them, 
electricity  generally  forms  the  intervening  link  or  middle 
term. 

Motion,  then,  will  directly  produce  heat  and  electricity, 
and  electricity,  being  produced  by  it,  will  produce  magnetism 
— a  force  which  is  always  developed  by  electrical  currents  at 
right  angles  to  the  direction  of  those  currents,  as  will  be  sub- 
sequently more  fully  explained.  Light  also  is  readily  pro- 
duced by  motion,  either  directly,  as  when  accompanying  the 
heat  of  friction,  or  mediately,  by  electricity  resulting  from 
motion  ;  as  in  the  electrical  spark,  which  has  most  of  the  at- 
tributes of  solar  light,  differing  from  it  only  in  those  respects 
in  which  light  differs  when  emanating  from  different  sources 
or  seen  through  different  media ;  for  instance,  in  the  position 
of  the  fixed  lines  in  the  spectrum  or  in  the  ratios  of  the  spaces 
occupied  by  rays  of  different  refrangibility.  In  the  decom- 
positions and  compositions  which  the  terminal  points  proceed- 
ing from  the  conductors  of  an  electrical  machine  develope 
when  immersed  in  different  chemical  media,  we  get  the  pro- 
duction of  chemical  affinity  by  electricity,  of  which  motion  is 
the  initial  source.  Lastly,  motion  may  be  again  reproduced 
by  the  forces  which  have  emanated  from  motion ;  thus,  the 


38  CORRELATION    OF   PHYSICAL   FORCES. 

divergence  of  the  electrometer,  the  revolution  of  the  electri- 
cal wheel,  the  deflection  of  the  magnetic  needle,  are,  when 
resulting  from  frictional  electricity,  palpable  movements  re- 
produced by  the  intermediate  modes  of  force,  which  have 
themselves  been  originated  by  motion. 


III.— HEAT. 

IF  we  now  take  HEAT  as  our  starting  point,  we  shall  find 
that  the  other  modes  of  force  may  be  readily  produced  by 
it.  To  take  motion  first :  this  is  so  generally,  I  think  I  may 
say  invariably,  the  immediate  effect  of  heat,  that  we  may  almost, 
if  not  entirely,  resolve  heat  into  motion,  and  view  it  as  a 
mechanically  repulsive  force,  a  force  antagonist  to  attraction 
of  cohesion  or  aggregation,  and  tending  to  move  the  particles 
of  all  bodies,  or  to  separate  them  from  each  other. 

It  may  be  well  here  to  premise,  that  in  using  the  terms 
1  particles  '  or  '  molecules/  which  will  be  frequently  employed 
in  this  Essay,  I  do  not  use  them  in  the  sense  of  the  atomist, 
or  mean  to  assert  that  matter  consists  of  indivisible  particles 
or  atoms.  The  words  will  .be  used  for  the  necessary  purpose 
of  contradistinguishing  the  action  of  the  indefinitely  minute  phy- 
sical elements  of  matter  from  that  of  masses  having  a  sensi- 
ble magnitude,  much  in  the  same  way  as  the  term  '  lines  '  or 
1  points '  may  be  used,  and  with  advantage  in  an  abstract 
sense  ;  though  there  does  not  exist,  in  fact,  a  thing  which  has 
length  and  breadth  without  thickness,  and  though  a  thing  with- 
out parts  or  dimensions  is  nothing. 

If  we  put  aside  the  sensation  which  heat  produces  in  our 
own  bodies,  and  regard  heat  simply  as  to  its  effects  upon  in- 
organic matter,  we  find  that,  with  a  very  few  exceptions,  which  I 


40  CORRELATION   OF   PHYSICAL   FORCES. 

shall  presently  notice,  the  effects  of  what  is  called  heat  are 
simply  an  expansion  of  the  matter  acted  upon,  and  that  the 
matter  so  expanded  has  the  power  by  its  own  contraction  of 
communicating  expansion  to  all'bodies  in  contiguity  with  it. 
Thus,  if  the  body  be  a  solid,  for  instance,  iron,  a  liquid,  say 
water,  or  a  gas,  say  atmospheric  air — each  of  these,  when 
heated,  is  expanded  in  every  direction ;  in  the  two  former 
cases,  by  increasing  the  heat  to  a  certain  point,  we  change 
the  physical  character  of  the  substance,  the  solid  becomes  a 
liquid,  and  the  liquid  becomes  a  gas ;  these,  however,  are 
still  expansions,  particularly  the  latter,  when,  at  a  certain 
period,  the  expansion  becomes  rapidly  and  indefinitely  greater. 
But  what  is,  in  fact,  commonly  done  in  order  to  heat  a  sub- 
stance, or  to  increase  the  heat  of  a  substance?  it  is  merely 
approximated  to  some  other  heated,  that  is,  to  some  other 
expanded  substance,  which  latter  is  cooled  or  contracted  as 
the  former  expands.  Let  us  now  divest  the  mind  of  the  impres- 
sion that  heat  is  in  itself  anything  substantive,  and  suppose 
that  these  phenomena  are  regarded  for  the  first  time,  and 
without  any  preconceived  notions  on  the  subject ;  let  us  in- 
troduce no  hypothesis,  but  merely  express  as  simply  as  we 
can  the  facts  of  which  we  have  become  cognisant ;  to  what 
do  they  amount  ?  to  this,  that  matter  has  pertaining  to  it  a 
molecular  repulsive  power,  a  power  of  dilatation,  which  is 
communicable  by  contiguity  or  proximity. 

Heat  thus  viewed,  is  motion,  and  this  molecular  motion 
we  may  readily  change  into  the  motion  of  masses,  or  motion 
in  its  most  ordinary  and  palpable  form :  for  example,  in  the 
steam  engine,  the  piston  and  all  its  concomitant  masses  of 
matter  are  moved  by  the  molecular  dilatation  of  the  vapour  of 
water. 

To  produce  continuous  motion  there  must  be  an  alternate 
action  of  heat  and  cold ;  a  given  portion  of  air,  for  instance, 
heated  beyond  the  temperature  of  the  circumambient  air,  is 
expanded.  If  now  it  be  made  to  act  on  a  movable  piston,  it 


HEAT.  4:1 

moves  this  to  a  point  at  which  the  tension  or  elastic  force  of 
the  confined  air  equals  that  of  the  surrounding  air.  If  the 
confined  air  be  kept  at  this  point,  the  piston  would  remain 
stationary ;  but  if  it  be  cooled,  the  external  air  exercising 
then  a  greater  relative  degree  of  pressure,  the  piston  returns 
towards  its  original  position  ;  just  as  it  will  be  seen,  when  we 
come  to  the  magnetic  force,  that  a  magnet  placed  in  a  partic- 
ular position  produces  motion  in  iron  near  it,  but  to  make 
this  motion  continuous,  or  to  obtain  an  available  mechanical 
power,  the  magnet  must  be  demagnetised,  or  a  stable  equili- 
brium is  obtained. 

In  the  case  of  the  piston  moved  by  heated  air  the  motion 
of  the  mass  becomes  the  exponent  of  the  amount  of  heat — 
i.  e.  of  the  expansion  or  separation  of  the  molecules ;  nor  do 
we,  by  any  of  our  ordinary  methods,  test  heat  in  any  other 
way  than  by  its  purely  dynamical  action.  The  various  modi- 
fications of  the  thermometer  and  pyrometer  are  all  measur- 
ers of  heat  by  motion :  in  these  instruments  liquid  or  solid 
bodies  are  expanded  and  elongated,  i.  e.  moved  in  a  definite 
direction,  and,  either  by  their  own  visible  motion,  or  by  the 
motion  of  an  attached  index,  communicate  to  our  senses  the 
amount  of  the  force  by  which  they  moved.  There  are,  in- 
deed, some  delicate  expe-riments  which  tend  to  prove  that  a 
repulsive  action  between  separate  masses  is  produced  by  heat. 
Fresnel  found  that  mobile  bodies  heated  in  an  exhausted  re- 
ceiver repelled  each  other  to  sensible  distances ;  and  Baden 
Powell  found  that  the  coloured  rings  usually  called  Newton's 
rings  change  their  breadth  and  position,  when  the  glasses  be- 
tween which  they  appear  are  heated,  in  a  manner  which 
showed  that  the  glasses  repelled  each  other.  M.  Faye's  the- 
ory of  comets  is  based  on  some  such  repellent  force.  There 
is,  however,  some  difficulty  in  presenting  these  phenomena  to 
the  mind  in  the  same  aspect  as  the  molecular  repulsive  action 
of  heat. 

The  phenomena  of  what  is  termed  latent  heat  have  been 


4:2  CORRELATION   OF   PHYSICAL  FORCES. 

generally  considered  as  strongly  in  favour  of  that  view  which 
regards  heat  either  as  actual  matter,  or,  at  all  events,  as  a 
substantive  entity,  and  not  a  motion  or  affection  of  ordinary 
matter. 

The  hypothesis  of  latent  matter  is,  I  venture  with  diffi- 
dence to  think,  a  dangerous  one — it  is  something  like  the  old 
principle  of  Phlogiston,  it  is  not  tangible,  visible,  audible ; 
it  is,  in  fact,  a  mere  subtle  mental  conception,  and  ought,  I 
submit,  only  to  be  received  on  the  ground  of  absolute  neces- 
sity, the  more  so  as  these  subtleties  are  apt  to  be  carried  on 
to  other  natural  phenomena,  and  so  they  add  to  the  hypothe- 
tical scaffolding  which  is  seldom  requisite,  and  should  be 
sparingly  used,  even  in  the  early  stages  of  discovery.  As  an 
instance,  I  think  a  striking  'one,  of  the  injurious  effects  of 
this,  I  will  mention  the  analogous  doctrine  of  '  invisible  light ; ' 
and  I  do  this,  meaning  no  disrespect  to  its  distinguished  au- 
thor, any  more  than  in  discussing  the  doctrine  of  latent  heat, 
I  can  be  supposed,  in  the  slightest  degree,  to  aim  at  detract- 
ing from  the  merits  of  the  illustrious  investigators  of  the  facts 
which  that  doctrine  seeks  to  explain.  Is  not '  invisible  light/ 
a  contradiction  in  terms  ?  has  not  light  ever  been  regarded  as 
that  agent  which  affects  our  visual  organs  ?  Invisible  light, 
then,  is  darkness,  and  if  it  exist,  then  is  darkness  light.  I 
know  it  may  be  said,  that  one  eye  can  detect  light  where 
another  cannot ;  that  a  cat  may  see  where  a  man  cannot ;  that 
an  insect  may  see  where  a  cat  cannot ;  but  then  it  is  not 
invisible  light  to  those  who  see  it :  the  light,  or  rather  the 
object  seen  by  the  cat,  may  be  invisible  to  the  man,  but  it 
is  visible  to  the  cat,  and,  therefore,  cannot  abstractedly  be 
said  to  be  invisible.  If  we  go  further,  and  find  an  agent 
which  affects  certain  substances  similarly  to  light,  but  does 
not,  as  far  as  we  are  aware,  affect  the  visual  organs  of  any 
animal,  then  is  it  not  an  erroneous  nomenclature  which  calls 
such  an  agent  light  ?  There  are  many  cases  in  which  a  de- 
viation from  the  once  accepted  meaning  of  words  has  so  grad- 


HEAT.  4:3 

ually  entered  into  common  usage  as  to  be  unavoidable,  but  I 
venture  to  think  that  additions  to  such  cases  should  as  far 
as  possible  be  avoided,  as  injurious  to  that  precision  of  lan- 
guage which  is  one  of  the  safest  guards  to  knowledge,  and 
from  the  absence  of  which  physical  science  has  materially 
suffered.  ' 

Let  us  now  shortly  examine  the  question  of  latent  heat, 
and  see  w.hether  the  phenomena  cannot  be  as  well,  if  not 
more  satisfactorily,  explained  without  the  hypothesis  of  la- 
tent matter,  an  idea  presenting  many  similar  difficulties  to 
that  of  invisible  light,  though  more  sanctioned  by  usage. 
Latent  heat  is  supposed  to  be  the  matter  of  heat,  associated, 
in  a  masked  or  dormant  state,  with  ordinary  matter,  not  ca- 
pable of  being  detected  by  any  test  so  long  as  the  matter  with 
which  it  is  associated  remains  in  the  same  physical  state,  but 
communicated  to  or  absorbed  from  other  bodies,  when  the 
matter  with  which  it  is  associated  changes  its  state.  To 
take  a  common  example :  a  pound  or  given  weight  of  water 
at  172°,  mixed  with  an  equal  weight  of  water  at  32°,  will 
acquire  a  mean  temperature,  or  102°  ;  while  water  at  172°, 
mixed  with  an  equal  weight  of  ice  at  32°,  will  be* reduced  to 
32°.  By  the  theory  of  latent  heat  this  phenomenon  is  thus 
explained: — In  the  first  case,  that  of  the  mixture  of  water 
with  water,  both  the  bodies  being  in  the  same  physical  state, 
no  latent  heat  is  rendered  sensible,  or  sensible  heat  latent ; 
but  in  the  second,  the  ice  changing  its  condition  from  the  solid 
to  the  liquid  state  abstracts  from  the  liquid  as  much  heat  as 
it  requires  to  maintain  it  in  the  liquid  state,  which  it  renders 
latent,  or  retains  associated  with  itself,  so  long  as  it  remains 
liquid,  but  of  which  heat  no  evidence  can  be  afforded  by  any 
thermoscopic  test. 

I  believe  this  and  similar  phenomena,  where  heat  is  con- 
nected with  a  change  of  state  >  may  be  explained  and  dis- 
tinctly comprehended  without  recourse  to  the  conception  of 
latent  heat,  though  it  requires  some  effort  of  the  mind  to  di- 


4A  CORRELATION   OF   PHYSICAL   FOECE3. 

vest  itself  of  this  idea,  and  to  view  the  phenomena  simply  in 
their  dynamical  relations.  To  assist  us  in  so  viewing  them, 
let  us  first  parallel  with  purely  mechanical  actions,  certain 
simple  effects  of  heat,  where  change  of  state  (I  mean  such 
change  as  from  the  solid  to  the  liquid,  or  liquid  to  the  gase- 
ous state)  is  not  concerned.  Thus,  place  within  a  receiver  a 
bladder,  and  heat  the  air  within  to  a  higher  temperature 
than  that  without  it,  the  bladder  expands ;  so,  force  the  air 
mechanically  into  it  by  the  air-pump,  the  bladder  expands  ; 
cool  the  air  on  the  outside,  or  remove  its  pressure  mechani- 
cally by  an  exhausting  pump,  the  bladder  also  expands  ;  con- 
versely, increase  the  external  repellent  force,  either  by  heat 
or  mechanical  pressure,  and  the  bladder  contracts.  In  the 
mechanical  effects,  the  force  which  produced  the  distension 
is  derived  from,  and  at  the  expense  of,  the  mechanical  power 
employed,  as  from  muscular  force,  from  gravitation,  from  the 
reacting  elasticity  of  springs,  or  any  similar  force  by  which 
the  air-pump  may  be  worked.  In  the  heating  effects,  the 
force  is  derived  from  the  chemical  action  in  the  lamp  or 
source  of  heat  employed. 

Let  us  next  consider  the  experiment  so  arranged  that  the 
force,  which  produces  expansion  in  the  one  case,  produces  a 
correlative  contraction  in  the  other :  thus,  if  two  bladders, 
with  a  connecting  neck  between  them,  be  half-filled  with  air, 
as  the  one  is  made  to  contract  by  pressure  the  other  will  di- 
late, and  vice  versa  ;  so  a  bladder  partly  filled  with  cold  air, 
and  contained  within  another  filled  with  hot  air,  expands, 
while  the  space  between  the  bladders  contracts,  exhibiting  a 
mere  transfer  of  the  same  amount  of  repulsive  force,  the 
mobility  of  the  particles,  or  their  mutual  attraction,  being 
the  same  in  each  body ;  in  other  words,  the  repulsive  force 
acts  in  the  direction  of  least  resistance  until  equilibrium  is 
produced ;  it  then  becomes  a  static  or  balanced,  instead  of  a 
dynamic  or  motive  force. 

Let   us   now   consider  the  case  where  a  solid  is  to  bo 


I 

HEAT.  45 

changed  to  a  liquid,  or  a  liquid  to  a  gas  ;  here  a  much  great- 
er amount  of  heat  or  repulsive  force  is  required,  on  account 
of  the  cohesion  of  the  particles  to  be  separated.  In  order  to 
separate  the  particles  of  the  solid,  precisely  as  much  force 
must  be  parted  with  by  the  warmer  liquid  body  as  keeps  an 
equal  quantity  of  it  in  its  liquid  state  ;  it  is,  indeed,  only  with 
a  more  striking  line  of  demarcation,  the  case  of  the  hot  and 
cold  bladder — a  part  of  the  repellent  power  of  the  hot  parti- 
cles is  transferred  to  the  cold  particles,  and  separates  them  in 
their  turn,  but  the  antagonist  force  of  cohesion  or  aggregation 
necessary  to  be  overcome,  being  in  this  case  much  stronger, 
requires  and  exhausts  an  exactly  proportionate  amount  of 
repellent  force  mechanically  to  overcome  it ;  hence  the  differ- 
ent effect  on  a  body  such  as  the  common  thermometer,  the 
expanding  liquid  of  which  does  not  undergo  a  similar  change 
of  state.  Thus,  in  the  example  above  given,  of  the  mixture 
of  cold  with  hot  water,  the  hot  and  cold  water  and  the 
mercury  of  the  thermometer  being  all  in  a  liquid  state  before, 
and  remaining  so  after  contact,  the  resulting  temperature  is 
an  exact  mean  ;  the  hot  water  contracts  to  a  certain  extent, 
the  cold  water  expands  to  the  same  extent,  and  the  ther- 
mometer either  sinks  or  rises  the  same  number  of  degrees, 
accordingly  as  it  had  been  previously  immersed  in  the  cold 
or  in  the  hot  solution,  its  mercury  gaining  or  losing  an  equiva- 
lent of  repellent  force.  In  the  second  instance,  viz.  the  mix- 
ture of  ice  with  hot  water,  the  substance  we  use  as  an  indi- 
cator, i.  e.  mercury,  does  not  undergo  the  same  physical 
change  as  those  whose  relations  of  volume  we  are  examining. 
The  force — viewing  heat  simply  as  mechanical  force — which 
is  employed  in  loosening  or  tearing  asunder  the  particles  of 
the  solid  ice,  is  abstracted  from  the  liquid  water,  and  from 
the  liquid  mercury  of  the  thermometer,  and  in  proportion  as 
this  fotce  meets  with  a  greater  resistance  in  separating 
the  particles  of  a  solid  than  of  a  liquid,  so  the  bodies 
which  yield  the  force  suffer  proportionately  a  greater  con- 
traction. 


46  CORRELATION    OF    PHYSICAL    FORCES. 

If  we  compare  the  action  of  heat  on  the  two  substances, 
water  and  mercury,  alone,  and  throw  out  of  our  consideration 
the  ice,  we  shall  be  able  to  apply  the  same  view :  thus,  if  a 
given  source  of  heat  be  applied  to  water  containing  a  mercu- 
rial thermometer,  both  the  water  and  mercury  gradually  ex- 
pand, but  in  different  degrees ;  at  a  certain  point  the  attrac- 
tive force  of  the  molecules  of  the  water  is  so  far  overcome 
that  the  water  becomes  vapour.  At  this  point,  the  heat  or 
force,  meeting  with  much  less  resistance  from  the  attraction 
of  the  particles  of  steam  than  from  those  of  the  mercury,  ex- 
pends itself  upon  the  former ;  the  mercury  does  not  further 
expand,  or  expands  in  an  infinitesimally  small  degree,  and 
the  steam  expands  greatly.  As  soon  as  this  arrives  at  a 
point  where  circumambient  pressure  causes  its  resistance  to 
further  expansion  to  be  equal  to  the  resistance  to  expansion 
in  the  mercury  of  the  thermometer,  the  latter  again  rises, 
and  so  both  go  on  expanding  in  an  inverse  ratio  to  their 
molecular  attractive  force.  If  the  circumambient  pressure  be 
increased,  as  by  confining  the  water  at  the  commencement 
of  the  experiment  within  a  less  expansible  body  than  itself, 
such  as  a  metallic  chamber,  then  the  mercury  of  the  ther- 
mometer continues  to  rise ;  and  if  the  experiment  were  con- 
tinued, the  water  being  confined  and  not  the  mercury,  until 
we  have  arrived  at  a  degree  of  repulsive  force  which  is  able 
to  overcome  the  cohesive  power  of  the  mercury,  so  that  this 
expands  into  vapour,  then  we  get  the  converse  effect ;  the 
force  expends  itself  upon  the  mercury,  which  expands  in- 
definitely, as  the  water  did  in  the  first  case,  and  the  water 
does  not  expand  at  all. 

Another  very  usual  mode  of  regarding  the  subject  may 
embarras  at  first  sight,  but  a  little  consideration  will  show 
that  it  is  explicable  by  the  same  doctrine.  Water  which  has 
ice  floating  in  it  will  give,  when  measured  by  the  thermo- 
meter, the  same  temperature  as  the  ice ;  i.  e.  both  the  water 
and  ice  contract  the  mercury  of  the  thermometer  to  the  point 


HEAT.  47 

conventionally  marked  as  32°.  It  may  be  said,  how  is  this 
reconcile  able  with  the  dynamical  doctrine,  for,  according  to 
that,  the  solid  should  take  from  the  mercury  of  the  ther- 
mometer more  repulsive  power  than  the  liquid ;  conse- 
quently, the  ice  should  contract  the  mercury  more  than  the 
water? 

My  answer  is,  that  in  the  proposition  as  thus  stated,  the 
quantities  of  the  water,  ice,  and  mercury  are  not  taken  into 
consideration,  and  hence  a  necessary  dynamical  element  is 
neglected:  if  the  element  of  quantity  be  included,  this  objec- 
tion will  not  apply.  Let  the  thermometer,  for  instance,  con- 
tain 13  oz.  of  mercury,  and  stand  at  100°  ;  if  placed  in  con- 
tact with  an  unlimited  quantity  of  ice  at  32°,  the  mercury 
will  sink  to  32°.  If  the  same  thermometer  be  immersed  in 
an  unlimited  quantity  of  water  at  32°,  the  mercury  sinks  also 
to  32°  ;  not  absolutely,  perhaps,  because,  however  great  the 
quantity  of  water  or  ice,  it  will  be  somewhat  raised  in  tem- 
perature by  the  warmer  mercury.  This  elevation  of  tempera- 
ture above  32°  will  be  smaller  in  proportion  as  the  quantity 
of  water  or  ice  is  larger  than  the  quantity  of  mercury ;  and, 
as  we  know  of  no  intermediate  state  between  ice  and  water, 
the  contact  of  a  thermometer  at  a  temperature  above  the 
freezing  point  with  any  quantity  of  ice  exactly  at  the  freezing 
point  would,  theoretically  speaking,  liquefy  the  whole,  pro- 
vided it  had  sufficient  time  ;  for  as  every  portion  of  that  ice 
would  in  time  have  its  temperature  raised  by  the  contact  of 
the  warmer  body,  and  as  any  elevation  of  temperature  above 
the  freezing  point  liquefies  ice,  every  portion  should  be  lique- 
fied. Practically  speaking,  however,  in  both  cases,  that  of 
the  water  and  of  the  ice,  when  the  quantity  is  indefinitely 
great  the  thermometer  falls  to  32° , 

Now  place  the  same  thermometer  at  100°,  successively 
in  one  oz.  of  water  at  32°,  and  in  one  of  ice  at  32°  ;  we  shall 
find  in  the  former  case  it  will  be  lowered  only  to  54°,  and  in 
the  latter  to  32°  ;  apply  to  this  the  doctrine  of  repulsive  force, 
and  we  get  a  satisfactory  explanation. 


4:8  CORRELATION   OF   PHYSICAL   FORCES. 

In  the  first  case,  the  quantities  both  of  ice  and  water  be- 
ing indefinitely  great  in  respect  to  the  mercury,  each  reduces 
it  to  its  own  temperature,  viz.  32°,  and  the  ice  cannot  reduce 
the  mercury  below  32°,  because  it  would  receive  back  repul- 
sive power  from  the  newly  formed  water,  and  this  would  be- 
come ice  ;  in  the  second  case,  where  the  quantities  are  limited, 
the  mercury  does  lose  more  repulsive  power  by  the  ice  than  by 
the  water,  and  the  observations  made  in  reference  to  the  first 
illustration  apply. 

The  above  doctrine  is  beautifully  instanced  in  the  experi- 
ment of  Thilorier,  by  which  carbonic  acid  is  solidified.  Car- 
bonic acid  gas,  retained  in  a  strong  vessel  under  great  pres- 
sure, is  allowed  to  escape  from  a  small  orifice  ;  the  sudden 
expansion  requires  so  great  a  supply  of  force,  that  in  furnish- 
ing the  demands  of  the  expanding  gas  certain  other  portions 
of  the  gas  contract  to  such  an  extent  as  to  solidify :  thus,  we 
have  reciprocal  expansion  and  contraction  going  on  in  one 
and  the  same  substance,  the  time  being  too  limited  for  the 
whole  to  assume  a  uniform  temperature,  or  in  other  words,  a 
uniform  extent  of  expansion. 

It  has  been  observed  with  reference  to  heat  thus  viewed, 
that  it  would  be  as  correct  to  say,  that  heat  is  absorbed,  or 
cold  produced  by  motion,  as  that  heat  is  produced  by  it.  This 
difficulty  ceases  when  the  mind  has  been  accustomed  to  re- 
gard heat  and  cold  as  themselves,  motion  ;  i.  e.  as  correlative 
expansions  and  contractions,  each  being  evidenced  by  relation, 
and  being  inconceivable  as  an  abstraction. 

For  instance,  if  the  piston  of  an  air-pump  be  drawn  down 
by  a  weight,  cold  is  produced  in  the  receiver.  It  may  be  here 
said  that  a  mechanical  force,  and  the  motion  consequent  upon 
it,  produces  cold ;  but  heat  is  produced  on  the  opposite  side 
of  the  piston,  if  a  receiver  be  adapted  so  as  to  retain  the  com- 
pressed air.  Assuming  them  to  equivalise  each  other,  the 
force  of  the  falling  weight  would  be  expressed  by  the  heat  of 
friction  of  the  piston  against  its  tube,  and  by  the  tension  or 


HEAT.  49 

power  of  reaction  of  the  compressed  against  the  dilated  air. 
If  the  heat  due  to  compression  be  made  to  perform  mechani- 
cal work,  it  would  pro  tanto  be  consumed,  and  could  not 
restore  the  temperature  to  the  dilated  air ;  but  if  it  perform 
no  work,  no  heat  is  lost.  Mr.  Joule  has  experimentally 
proved  this  proposition. 

In  commencing  the  subject  of  heat,  I  asked  my  reader  to 
put  out  of  consideration  the  sensations  which  heat  produces 
in  our  own  bodies ;  I  did  this  because  these  sensations  are 
likely  to  deceive,  and  have  deceived  many  as  to  the  nature  of 
heat.  These  sensations  are  themselves  occasioned  by  simi- 
lar expansions  to  those  which  we  have  been  considering  ;  the 
liquids  of  the  body  are  expanded,  i.  e.  rendered  less  viscid  by 
heat,  and  from  their  more  ready  flow,  we  obtain  the  sensation 
of  agreeable  warmth.  By  a  greater  degree  of  heat,  their  ex- 
pansion becomes  too  great,  giving  rise  to  a  sense  of  pain,  and, 
if  pushed  to  extremity,  as  with  the  heat  which  produces  a 
burn,  the  liquids  of  the  body  are  dissipated  in  vapour,  and  an 
injury  or  destruction  of  the  organic  structure  takes  place.  A 
similar  though  converse  effect  may  be  produced  by  intense 
cold ;  the  application  of  frozen  mercury  to  the  animal  body 
produces  a  burn  similar  to  that  produced  by  great  heat,  and 
accompanied  with  a  similar  sensation. 

Doubtless  other  actions  than  those  above  mentioned  inter- 
fere in  producing  the  sensations  of  heat  and  cold  ;  but  I  think 
it  will  be  seen  that  these  will  not  affect  the  arguments  as  to 
the  nature  of  heat.  The  phenomenal  effects  will  be  found 
unaltered :  heat  will  still  be  found  to  be  expansion,  cold  to  be 
contraction  ;  and  the  expansion  and  contraction  are,  as  with 
the  two  bladders  of  air,  correlative — i.  e.  we  cannot  expand 
one  body,  A,  without  contracting  some  other  body,  B  ;  we 
cannot  contract  A  without  expanding  B,  assuming  that  we  view 
the  bodies  with  relation  to  heat  alone,  and  suppose  no  other 
force  to  be  manifested. 

I  have  said  that  there  are  few  exceptions  as  to  heat  being 
3 


50  CORRELATION   OF   PHYSICAL   FORCES. 

always  manifested  by  an  expansion  of  matter.  One  class  of 
these  exceptions  is  only  apparent :  moist  clay,  animal  or 
vegetable  fibre,  and  other  substances  of  a  mixed  nature, 
which  contain  matter  of  different  characters,  some  of  which  is 
more  and  some  less  volatile,  i.  e.  expansible,  are  contracted 
on  the  application  of  heat ;  this  arises  from  the  more  volatile 
matter  being  dissipated  in  the  form  of  vapour  or  gas  ;  and  the 
interstices  of  the  less  volatile  being  thus  emptied,  the  latter 
contracts  by  its  own  cohesive  attraction,  giving  thus  a  prima 
facie  appearance  of  contraction  by  heat.  The  pyrometer  of 
Wedgwood  is  explicable  on  this  principle. 

The  second  class  of  exceptions,  though  much  more  limited 
in  extent,  is  less  easily  explained.  Water,  fused  bismuth, 
and  probably  some  other  substance*  (though  the  fact  as  to 
them  is  not  clearly  established),  expand  as  they  approach 
very  near  to  the  freezing  or  solidifying  point.  The  most 
probable  explanation  of  these  exceptions  is,  that  at  the  point 
of  maximum  density  the  molecules  of  these  bodies  assume  a 
polar  or  crystalline  condition ;  that  by  the  particles  being 
thus  arranged  in  linear  directions  like  chevaux  de  frise, 
interstitial  spaces  are  left,  containing  matter  of  less  den- 
sity, so  that  the  specific  density  of  the  whole  mass  is  dimin- 
ished. 

Some  recent  experiments  of  Dr.  Tyndall  on  the  physical 
properties  of  ice  seem  to  favour  this  view.  When  a  sun- 
beam, concentrated  by  a  lens,  is  allowed  to  fall  on  a  piece 
of  apparently  homogeneous  ice  the  path  of  the  rays  is  in- 
stantly studded  with  numerous  luminous  spots  like  minute  air 
bubbles,  and  the  planes  of  freezing  are  made  manifest  by 
these  and  by  small  fissures.  Stars  or  flower-like  figures  of 
six  petals  appear  parallel  to  the  planes  of  freezing,  and  seem- 
ingly spreading  out  from  a  central  bubble.  These  flowers 
are  formed  of  water.  When  the  ice  is  melted  in  warm  water 
no  air  is  given  off  from  the  bubbles,  so  they  seem  to  be  va- 
cuous ;  it  is,  however,  possible  that  extremely  minute  parti- 


HEAT.  51 

cles  of  air  sufficient  to  form  foci  for  the  melting  points  of  ice 
might  be  dissolved  by  the  water  as  soon  as  they  came  in  con- 
tact with  it.  Be  this  as  it  may,  the  existence  of  these  points 
throughout  the  ice,  where  it  gives  way  to  the  heat  of  the  solar 
beam,  if  it  does  not  prove  actual  vacuous  or  aeriform  spaces 
to  exist  in  ice,  proves  that  it  is  not  homogeneous,  that  its 
structure  is  probably  definitely  crystalline,  and  that  the 
matter  composing  it  is  in  different  degrees  of  aggregation,  so 
that  its  mean  specific  gravity  might  well  be  less  than  that  of 
water. 

"We  cannot  examine  piecemeal  the  ultimate  structure  of 
matter,  but  in  addition  to  the  fact  that  the  bodies  which 
evince  this  peculiarity  are  bodies  which,  when  solidified,  ex- 
hibit a  very  marked  crystalline  character,  there  are  experi- 
ments which  show  that  water  between  the  point  of  maximum 
density  and  its  point  of  solidification  polarises  light  circularly  ; 
showing,  if  these  experiments  be  correct,  a  structural  altera- 
tion in  water,  and  one  analogous  to  that  possessed  by  certain 
crystalline  solids,  and  to  that  possessed  by  water  itself,  where 
it  is  forcibly  made  to  assume  a  polarised  condition  by  the  in- 
fluence of  magnetism. 

The  accuracy  of  these  results  has,  however,  been  doubted, 
and  the  experiments  have  not  succeeded  when  repeated  by 
very  experienced  hands.  Whether  this  be  so  or  not,  and 
whether  the  above  explanation  of  the  exception  to  the  other- 
wise invariable  effect  of  expansion  by  heat  be  or  be  not  re- 
garded as  admissible,  must  be  left  to  the  judgment  of  each 
individual  who  thinks  upon  the  subject ;  at  all  events,  no 
theory  of  heat  yet  proposed  removes  the  difficulty,  and  there- 
fore it  equally  opposes  every  other  view  of  the  phenom- 
ena of.  heat,  as  it  does  that  which  I  have  here  consid- 
ered, and  which  regards  heat  as  communicable  expansive 
force. 

As  certain  bodies  expand  in  freezing,  and  indeed,  under 
some  circumstances,  before  they  arrive  at  the  temperature  at 


52  CORRELATION   OF   PHYSICAL  FORCES. 

which  they  solidify,  we  get  the  apparent  anomaly  that  the 
motion  or  mechanical  force  generated  by  heat  or  change  of 
temperature  is  reversed  in  direction  when  we  arrive  at  the 
point  of  change  from  the  solid  to  the  liquid  state.  Thus  a 
piece  of  ice  at  the  temperature  of  Zero,  Fahrenheit,  would 
expand  by  heat,  and  produce  a  mechanical  force  by  such  ex- 
pansion until  it  arrives  at  32°  ;  but  then  by  an  increment  of 
heat  it  contracts,  and  if  the  first  expansion  had  moved  a  pis- 
ton upwards,  the  subsequent  contraction  would  bring  it  back 
to  a  certain  extent,  or  move  it  downwards,  an  apparent  nega- 
tion of  the  force  of  heat. 

Again  with  water  above  40°,  i.  e,  above  its  point  of 
maximum  density,  a  progressive  increment  of  cold  or  decre- 
ment of  heat  would  produce  contraction  to  a  certain  point, 
and  then  expansion  or  a  mechanical  force  in  an  opposite  direc- 
tion. Thus  not  only  heat  or  the  expansive  force  given  to 
other  bodies  by  a  body  cooling  would  be  given  out  by  water 
freezing,  but  also  the  force  due  to  the  converse  expansion  in  the 
body  itself,  and  force  would  thus  seem  to  be  got  out  of  noth- 
ing :  but  if  water  in  a  confined  space  be  gradually  cooled,  the 
expansion  attendant  on  its  cooling  as  it  approaches  the  freez- 
ing point  would  occasion  pressure  amongst  its  particles,  and 
thence  tend  to  antagonise  the  force  of  dilatation  produced  in 
them  by  cooling,  or  to  resist  their  tendency  to  freeze  ;  or  in 
other  words,  the  pressure  would  tend  to  liquefaction,  and  con- 
versely to  the  usual  effect  of  pressure,  produce  cold  instead 
of  heat,  and  thus  neutralise  some  of  the  heat  yielded  by  the 
cooling  body.  Hence  we  find  that  it  requires  a  lower  tem- 
perature to  freeze  water  under  pressure  than  when  exempt 
from  it,  or  that  the  freezing  point  is  lowered  as  the  pressure 
increases  for  bodies  which  expand  in  freezing — an  effect  first 
predicted  by  Mr.  J.  Thompson,  and  experimentally  verified 
by  Mr.  W.  Thompson  ;  while  as  shown  by  M.  Bunsen,  the 
converse  effect  takes  place  with  bodies  which  contract  in 
freezing.  Here  the  pressure  cooperates  with  the  effects  of 


HEAT.  53 

cold,  both  tending  to  approximate  the  particles,  and  such  sub- 
stances solidify  at  a  higher  temperature  in  proportion  as  the 
pressure  is  greater ;  so  that  we  might  expect  a  body  of  this 
class,  which  under  the  ordinary  pressure  of  the  air  is  at  a 
temperature  just  above  its  freezing  point,  to  solidify  by 
being  submitted  to  pressure  alone,  the  temperature  being 
kept  constant. 

A  similar  class  of  exception  to  the  general  effect  of 
heat  in  expanding  bodies  is  presented  by  vulcanised  caout- 
chouc. This  has  been  observed  by  Mr.  Gough,  and,  in- 
deed, was  pointed  out  to  me  many  years  ago  by  Mr. 
Brockedon  to  be  heated  when  stretched,  and  cooled  when 
unstretched. 

Mr.  Joule  finds  that  its  specific  gravity  is  lower  when 
stretched  than  when  unstretched,  and  that  when  heated 
in  its  stretched  state  it  shortens,  presenting  in  this  par- 
ticular condition  a  similar  series  of  converse  relations  to 
those  which  are  presented  by  water  near  or  at  its  freezing 
point. 

With  the  exception  of  this  class  of  phenomena,  which 
offer  difficulties  to  any  theory  which  has  been  proposed,  the 
general  phenomena  of  heat  may,  I  believe,  be  explained  upon 
a  purely  dynamical  view,  and  more  satisfactorily  than  by 
having  recourse  to  the  hypothesis  of  latent  matter.  Many, 
however,  of  the  phenomena  of  heat  are  involved  in  much 
mystery,  particularly  those  connected  with  specific  heat  or 
that  relative  proportion  of  heat  which  equal  weights  of  differ- 
ent bodies  require  to  raise  them  from  a  given  temperature  to 
another  given  temperature,  which  appear  to  depend  in  some 
way  hitherto  inexplicable  upon  the  molecular  constitution  of 
different  bodies. 

The  view  of  heat  which  I  have  taken,  viz.  to  regard  it 
simply  as  a  communicable  molecular  repulsive  force,  is  sup- 
ported by  many  of  the  phenomena  to  which  the  term  specific 
or  relative  heat  is  applied ;  for  example,  bodies  as  they  in- 


54:  CORRELATION   OF   PHYSICAL   FORCES. 

crease  in  temperature  increase  in  specific  heat.  The  ratio 
of  this  increase  in  specific  heat  is  greater  with  solids  than 
with  liquids,  although  the,  latter  are  more  dilatable  ;  an 
effect  probably  depending  upon  the  commencement  of  fusion. 
Again,  those  metals  whose  rate  of  expansion  increases  most 
rapidly  when  they  are  heated,  increase  most  in  specific  heat ; 
and  their  specific  heat  is  reduced  by  percussion,  which,  by 
approximating  their  particles,  makes  them  specifically  more 
dense.  When,  however,  we  examine  substances  of  very 
different  physical  characters,  we  find  that  their  specific  heats 
have  no  relation  to  their  density  or  rate  of  expansion  by 
heat ;  their  differences  of  specific  heat  must  depend  upon 
their  intimate  molecular  constitution  in  a  manner  accounted 
for  (as  far  as  I  am  aware)  by  no  theory  of  heat  hitherto 
proposed. 

In  the  greater  number,  probably  in  all  solids  and  liquids, 
the  expansion  by  heat  is  relatively  greater  as  the  temperature 
is  higher ;  or,  preserving  the  view  of  expansion  and  contrac- 
tion, if  two  equal  portions  of  the  same  substance  be  juxta- 
posed at  different  temperatures,  the  hotter  portion  will  con- 
tract a  little  more  than  the  colder  will  expand ;  from  this 
fact,  viz.  that  the  coefficient  of  expansion  increases  in  a  given 
body  with  the  temperature,  and  from  other  considerations, 
Dr.  Wood  has  argued,  with  much  apparent  reason,  that  the 
nearer  the  particles  of  bodies  are  to  each  other,  the  less  they 
require  to  move  to  produce  a  given  expansion  or  contraction 
in  those  of  another  body.  His  mode  of  reasoning,  if  I  rightly 
conceive  it,  may  be  concisely  put  as  follows  : — 

As  bodies  contract  by  cold,  it  is  clear  that,  in  a  given 
body,  the  lower  the  temperature  the  nearer  are  the  particles  ; 
and,  as  the  coefficient  of  expansion  increases  with  the  tem- 
perature, the  lower  the  temperature  of  the  substance  be,  the 
less  the  particles  require  to  move,  or  approach  to  or  recede 
from  each  other,  so  as  to  compensate  the  correlative  recession 
or  approach  of  the  particles  in  a  hotter  portion  of  the  same 


HEAT.  55 

substance,  that  is,  in  another  portion  of  the  same  substance 
in  which  the  particles  are  more  distant  from  each  other. 
The  amount  of  approximation  or  recession  of  the  particles  of 
a  body,  in  other  words,  its  change  of  bulk  by  a  given  change 
of  temperature,  being  thus  in  a  given  substance  an  index  of 
the  relative  proximity  of  its  particles,  may  it  not  be  so  of  all 
bodies  ?  The  proposition  is  very  ingeniously  argued  by  Dr. 
Wood,  but  the  argument  is  based  upon  certain  hypotheses  as 
to  the  sizes  and  distances  of  atoms,  which  must  be  admitted 
as  postulates  by  those  who  adopt  his  conclusions.  Dr. 
Wood  seeks  by  means  of  this  theory  to  explain  the  heat  pro- 
duced by  chemical  combination,  and  I  shall  endeavour  to  give 
a  sketch  of  his  mode  of  reasoning  when  I  arrfve  at  that  part 
of  my  subject. 

Although  the  comparative  effects  of  specific  heat  may  not 
be  satisfactorily  explicable  by  any  known  theory,  the  absolute 
effect  of  heat  upon  each  separate  substance  is  simply  expan- 
sion, but  when  bodies  differing  in  their  physical  characters 
are  used,  the  rate  of  expansion  varies,  if  measured  by  the 
correlative  contractions  exhibited  by  the  substances  produc- 
ing it.  Though  I  am  obliged,  in  order  to  be  intelligible,  to 
talk  of  heat  as  an  entity,  and  of  its  conduction,  radiation,  &c., 
yet  these  expressions  are,  in  fact,  inconsistent  with  the  dyna- 
mic theory  which  regards  heat  as  motion  and  nothing  else  ; 
thus  conduction  would  be  simply  a  progressive  dilatation  or 
motion  of  the  particles  of  the  conducting  substance,  radiation 
an  undulation  or  motion  of  the  particles  of  the  medium 
through  which  the  heat  is  said  to  be  transmitted,  &c. ;  and  it 
is  a  strong  argument  in  favour  of  this  theory,  that  for  every 
diversity  in  the  physical  character  of  bodies,  and  for  every 
change  in  the  structure  and  arrangement  of  particles  of  the 
same  body,  a  change  is  apparent  in  the  thermal  effects. 
Thus  gold  conducts  heat,  or  transmits  the  motion  called  heat, 
more  readily  than  copper,  copper  than  iron,  iron  than  lead, 
and  lead  than  porcelain,  &c. 


56  CORRELATION   OF   PHYSICAL   FORCES, 

So  when  the  structure  of  a  substance  is  not  homogeneous, 
we  have  a  change  in  the  conduction  of  different  parts  depend- 
ent upon  the  structure.  This  is  beautifully  shown  with 
bodies  whose  structure  is  symmetrically  arranged,  as  in  crys- 
tals. Senarmont  has  shown  that  crystals  conduct  heat  differ- 
ently in  different  directions  with  reference  to  the  axis  of 
symmetry,  but  definitely  in  definite  directions.  His  mode  of 
experimenting  is  as  follows  : — A  plate  of  the  crystal  is  cut  in 
a  direction,  for  one  set  of  experiments  parallel,  and  for 
another  at  right  angles  to  the  axis  ;  a  tube  of  platinum  is  in- 
serted through  the  centre  of  the  plate,  and  bent  at  one 
extremity,  so  as  to  be  capable  of  being  heated  by  a  lamp 
without  the  heat  which  radiates  from  the  lamp  affecting  the 
crystal ;  the  surfaces  or  bases  of  the  plate  of  crystal  are 
covered  with  wax.  When  the  platinum  is  heated,  the  direc- 
tion of  the  heat  conducted  by  the  crystal  is  made  known  by 
the  melting  of  the  wax,  and  a  curved  line  is  visible  at  the 
juncture  of  the  solid  and  liquid  wax.  This  curve,  with 
homogeneous  substances,  as  glass  or  zinc,  is  a  circle ;  it  is 
also  a  circle  on  plates  of  calc  spar  cut  perpendicular  to  th& 
axis  of  symmetry ;  but  on  plafes  cut  parallel  to  the  axis  of 
symmetry,  and  having  their  plane  perpendicular  to  one  of  the 
faces  of  the  primitive  rhombohedron,  the  curves  are  well- 
defined  ellipses,  having  their  longer  axes  in  the  direction  of 
the  axis  of  symmetry,  showing  that  this  axis  is  a  direction  of 
greater  conductibility.  From  experiments  of  this  character 
the  inference  is  drawn,  that  *  in  media  constituted  like  crys- 
tals of  the  rhombohedral  system,  the  conducting  power  varies 
in  such  a  manner,  that,  supposing  a  centre  of  heat  to  exist 
within  them,  and  the  medium  to  be  indefinitely  extended  in 
all  directions,  the  isothermal  surfaces  are  concentric  ellipsoids 
of  revolution  round  the  axis  of  symmetry,  or  at  least  surfaces 
differing  but  little  therefrom.' 

Knoblauch  has  further  shown,  that  radiant  heat  is  absorb- 
ed in  different  degrees,  according  as  its  direction  is  parallel 
or  perpendicular  to  the  axis  of  a  crystal. 


HEAT.  57 

If  we  select  a  substance  of  a  different  but  also  of  a  definite 
structure-,  such  as  wood,  we  find  that  heat  progresses  through 
it  with  more  or  less  rapidity,  according  to  its  direction  with 
reference  to  the  fibre  of  the  wood :  thus  Decandolle  and  De 
la  Rive  found  that  the  conduction  was  better  in  a  direction 
parallel  to  the  fibre  than  in  one  transverse  to  it ;  and  Dr. 
Tyndall  has  added  the  fact,  that  the  conduction  is  better  in  a 
direction  transverse  to  the  fibres  and  layers  of  the  wood  than 
when  transverse  to  the  fibre  but  parallel  to  the  layers,  though 
in  both  these  directions  the  conduction  is  inferior  to  that  fol- 
lowing the  direction  of  the  fibre.  Thus,  in  the  three  possible 
directions  in  which  the  structure  of  wood  may  be  contem- 
plated, we  have  three  different  degrees  of  progression  for 
heat. 

In  the  above  examples  we  see,  as  we  shall  see  farther  on 
with  reference  to  all  the  so-called  imponderables,  that  the 
phenomena  depend  upon  the  molecular  structure  of  the  mat- 
ter affected ;  and  although  these  facts  are  not  absolutely  in- 
consistent with  the  theory  which  supposes  them  to  be  fluids 
or  entities,  it  will,  I  think,  be  found  to  be  far  more  consistent 
with  that  which  views  them  as  motion.  Heat,  which  we  are 
at  present  considering,  cannot  be  insulated:  we  cannot  re- 
move the  heat  from  a  substance  and  retain  it  as  heat ;  we 
can  only  transmit  it  to  another  substance,  either  as  heat  or 
as  some  other  mode  of  force.  We  only  know  certain  changes 
of  matter,  for  which  changes  heat  is  a  generic  name ;  the 
thing  heat  is  unknown. 

Heat  having  been  shown  to  be  a  force  capable  of  pro- 
ducing motion,  and  motion  to  be  capable  of  producing  the 
other  modes  of  force,  it  necessarily  follows  that  heat  is  capa- 
ble, mediately,  of  producing  them ;  I  will,  therefore,  content 
myself  with  enquiring  how  far  heat  is  capable  of  immediately 
producing  the  other  modes  of  force.  It  will  immediately 
produce  electricity,  as  shown  in  the  beautiful  experiments  of 
Seebeck,  one  of  which  I  have  already  cited,  which  experi- 
3* 


58  CORRELATION   OF  PHYSICAL  FORCES. 

ments  proved,  that  when  dissimilar  metals  are  made  to  touch, 
or  are  soldered  together  and  heated  at  the  point  of  contact,  a 
current  of  electricity  flows  through  the  metals  having  a  defi- 
nite direction  according  to  the  metals  employed,  which  cur- 
rent continues  as  long  as  an  increasing  temperature  is  grad- 
ually pervading  the  metals,  ceases  when  the  temperature  is 
stationary,  and  flows  in  the  contrary  direction  with  the  decre- 
ment of  temperature. 

Another  class  of  phenomena  which  have  been  generally 
attributed  to  the  effects  of  radiant  heat,  and  to  which,  from 
this  belief,  the  term  thermography  has  been  applied,  may 
also,  in  their  turn,  be  made  to  exhibit  electrical  effects — ef- 
fects here  of  Franklinic  or  static  electricity,  as  Seebeck's  ex- 
periments showed  effects  of  voltaic  or  dynamic  electricity. 

If  polished  discs  of  dissimilar  metals — say,  zinc  and  cop- 
per— be  brought  into  close  proximity,  and  kept  there  for 
some  time,  and  either  of  them  has  irregularities  upon  its  sur- 
face, a  superficial  outline  of  these  irregularities  is  traceable 
upon  the  other  disc,  and  vice  versa.  Many  theories  have 
been  framed  to  account  for  this  phenomenon,  but  whether  it 
be  due  or  not  to  thermic  radiations,  -the  relative  temperature 
of  the  discs,  their  relative  capacities  and  conducting  and 
radiating  powers  for  heat,  undoubtedly  influence  the  phe- 
nomena. 

Now,  if  two  such  discs  in  close  proximity  be  connected 
with  a  delicate  electroscope,  and  then  suddenly  separated, 
the  electroscope  is  affected,  showing  that  the  reciprocal  ra- 
diation from  surface  to  surface  has  produced  electrical  force. 
I  cite  this  experiment  in  treating  of  heat  as  an  initial  force, 
because  at  present  the  probabilities  are  in  favour  of  thermic 
radiation  producing  the  phenomenon.  The  origin  of  these 
so-called  thermographic  effects  is,  however,  a  question  open 
to  much  doubt,  and  needs  much  further  experiment.  When 
I  first  published  the  experiment  which  showed  that  the  mere 
approximation  of  metallic  discs  would  give  rise  to  electrical 


HEAT.  59 

effects,  I  mentioned  that  I  considered  the  fact  of  the  superfi- 
cial change  upon  the  surface  of  metals  in  proximity,  and,  a 
fortiori^  in  contact,  would  explain  the  developement  of  elec- 
tricity in  Volta's  original  contact  experiment,  without  having 
recourse  to  the  contact  theory,  i.  e.  a  theory  which  supposes 
a  force  to  be  produced  by  mere  contact  of  dissimilar  metals 
without  any  molecular  or  chemical  change.  I  have  seen 
nothing  to  alter  this  view.  Mr.  Gassiot  has  repeated  and 
verified  my  experiment  with  more  delicate  apparatus  and 
under  more  unexceptionable  circumstances  ;  and  without  say- 
ing that  radiant  heat  is  the  initial  force  in  this  case,  we  have 
evidence,  by  the  superficial  change  which  takes  place  in 
bodies  closely  approximated,  that  some  molecular  change  is 
taking  place,  some  force  is  called  into  action  by  their  proxim- 
ity, which  produces  changes  in  matter  as  it  expends,  or 
rather  transmits  itself;  and,  therefore,  is  not  a  force  without 
molecular  change,  as  the  supposed  contact  force  would  be. 
The  force  in  this,  as  in  all  other  cases,  is  not  created,  but  de- 
veloped by  the  action  of  matter  on  matter,  and  not  annihi- 
lated, as  it  is  shown  by  this  experiment  to  be  convertible  into 
another  mode  of  force. 

To  say  that  heat  will  produce  light,  is  to  assert  a  fact  ap- 
parently familiar  to  every  one,  but  there  may  be  some  rea- 
son to  doubt  whether  the  expression  to  produce  light  is  cor- 
rect in  this  particular  application  ;  the  relation  between  heat 
and  light  is  not  analogous  to  the  correlation  between  these 
and  the  other  four  affections  of  matter.  Heat  and  light  ap- 
pear to  be  rather  modifications  of  the  same  force  than  dis- 
tinct forces  mutually  dependent.  The  modes  of  action  of  ra- 
diant heat  and  of  light  are  so  similar,  both  being  subject  to 
the  same  laws  of  reflection,  refraction,  and  double  refraction, 
and  polarisation,  that  their  difference  appears  to  exist  more 
in  the  manner  in  which  they  affect  our  senses  than  in  our 
mental  conception  of  them. 

The  experiments  of  Melloni,  which  have  mainly  contrib- 


60  CORRELATION   OF  PHYSICAL   FORCES. 

uted  to  demonstrate  this  close  analogy  of  heat  and  light,  af- 
ford a  beautiful  instance  of  the  assistance  which  the  progress 
of  one  branch  of  physical  science  renders  to  that  of  another. 
The  discoveries  of  Oersted  and  Seebeck  led  to  the  construc- 
tion of  an  instrument  for  measuring  temperature,  incompara- 
bly more  delicate  than  any  previously  known.  To  distin- 
guish it  from  the  ordinary  thermometer,  this  instrument  is 
called  the  thermomultiplier.  It  consists  of  a  series  of  small 
bars  of  bismuth  and  antimony,  forming  one  zigzag  chain  of 
alternations  arranged  parallel  to  each  other,  in  the  shape  of 
a  cylinder  or  prism  ;  so  that  the  points  of  junction,  which  are 
soldered,  shall  be  all  exposed  at  the  bases  of  the  cylinder : 
the  two  extremities  of  this  series  ar,e  united  to  a  galvano- 
meter— that  is,  a  flat  coil  of  wire  surrounding  a  freely-sus- 
pended magnetic  needle,  the  direction  of  which  is  parallel  to 
the  convolutions  of  the  wire.  When  radiant  heat  impinges 
upon  the  soldered  ends  of  the  multiplier,  a  thermo-electric 
current  is  induced  in  each  pair ;  and,  as  all  these  currents 
tend  to  circulate  in  the  same  direction,  the  energy  of  the 
whole  is  increased  by  the  cooperating  forces :  this  current, 
traversing  the  helix  of  the  galvanometer,  deflects  the  needle 
from  parallelism  by  virtue  of  the  electro-magnetic  tangential 
force,  and  the  degree  of  this  deflection  serves  as  the  index 
of  the  temperature. 

Bodies  examined  by  these  means  show  a  remarkable  dif- 
ference between  their  transcalescence,  or  power  of  transmit- 
ting heat,  and  their  transparency :  thus,  perfectly  transparent 
alum  arrests  more  heat  than  quartz  so  dark  coloured  as  to  be 
opaque  ;  and  alum  coupled  with  green  glass  Melloni  found 
was  capable  of  transmitting  a  beam  of  brilliant  light,  while, 
with  the  most  delicate  thermoscope,  he  could  detect  no  indi- 
cations of  transmitted  heat :  on  the  other  hand,  rock-salt,  the 
most  transcalescent  body  known,  may  be  covered  with  soot 
until  perfectly  opaque,  and  yet  be  found  capable  of  transmit- 
ting a  considerable  quantity  of  heat.  Radiant  heat,  when 


HEAT.  61 

transmitted  through  a  prism  of  rock-salt,  is  found  to  be  une- 
qually refracted,  as  is  the  case  with  light ;  and  the  rays  of 
heat  thus  elongated  into  what  is,  for  the  sake  of  analogy, 
called  a  spectrum,  are  found  to  possess  similar  properties  to 
the  primary  or  coloured  rays  of  light.  Thus  rock-salt  is  to 
heat  what  colourless  glass  is  to  light ;  it  transmits  heat  of  all 
degrees  of  refrangibility :  alum  is  to  heat  as  red  glass  to 
light ;  it  transmits  the  least,  and  stops  the  most  refrangible 
rays  ;  and  rock-salt  covered  with  soot  represents  blue  glass, 
transmitting  the  most,  and  stopping  the  least  refrangible  rays. 

Certain  bodies,  again,  reflect  heat  of  different  refrangi- 
bility:  thus  paper,  snow,  and  lime,  although  perfectly  white 
— that  is,  reflecting  light  of  all  degrees  of  refrangibility,  re- 
flect heat  only  of  certain  degrees  ;  while  metals,  which  are 
coloured  bodies — that  is,  bodies  which  reflect  light  only  of 
certain  degrees  of  refrangibility — reflect  heat  of  all  degrees. 
Radiant  heat  incident  upon  substances  which  doubly  refract 
light  is  doubly  refracted ;  and  the  emergent  rays  are  polar- 
ised in  planes  at  right  angles  to  each  other,  as  is  the  case 
with  light. 

The  relation  of  radiation  to  absorption  also  holds  good 
with  light  as  with  heat:  with  the  latter  it  has  been  long 
known  that  the  radiating  power  of  different  substances  is  di 
rectly  proportional  to  their  absorptive  and  inverse  to  their 
reflective  power  ;  or  rather,  that  the  sum  of  the  heat  radia 
ted  and  reflected  is  a  constant  quantity.     So,  as  has  been 
shown  by  Mr.  Balfour  Stewart,  the   absorption  bears  the 
same  relation  to  radiation  for  heat  as  to  quality  as  well  as 
quantity. 

Light  presents  us  with  similar  relations.  Coloured  glass, 
when  heated  so  as  to  be  luminous,  emits  the  same  light 
which  at  ordinary  temperatures  it  absorbs:  thus  red  glass 
gives  out  or  radiates  a  greenish  light,  and  green  glass  a  red 
tint. 

The  flame  of  substances  containing  sodium  yields  a  yel- 


02  CORRELATION   OF   PHYSICAL   FORCES. 

low  light  of  such  purity  that  other  colours  exposed  to  it  ap- 
pear black — a  phenomenon  shown  by  the  familiar  experiment 
of  exposing  a  picture  of  bright  colours,  other  than  yellow,  to 
the  flame  of  spirits  of  wine  with  which  common  salt  is  mixed : 
the  picture  loses  its  colours,  and  appears  to  be  black  and 
white.  When  the  prismatic  spectrum  of  such  a  flame  is  ex- 
amined, it  is  found  to  exhibit  two  bright  yellow  lines  at  a  cer- 
tain fixed  position.  If  a  source  of  light  be  employed  which 
gives  no  lines  in  its  spectrum,  and  this,  being  at  a  higher 
temperature,  be  made  to  pass  through  the  sodium  flame,  two 
dark  lines  will  appear  in  the  spectrum  precisely  coincident 
in  position  with  the  yellow  lines  which  were  given  by  the  so- 
dium flame  itself.  The  same  relation  of  absorption  to  radia- 
tion is  therefore  shown  here  :  the  substance  absorbs  that 
light  which  it  yields  when  it  is  itself  the  source  of  light. 
The  same  is  true  of  other  substances,  the  spectra  of  which 
exhibit  respectively  lines  of  peculiar  colour  and  position. 
Now,  the  solar  prismatic  spectrum  is  traversed  by  a  great 
number  of  dark  lines ;  and  Kirchoff  has  deduced  from  con- 
siderations such  as  those  which  I  have  shortly  stated,  that 
these  dark  lines  in  the  solar  spectrum  are  due  to  metals  ex- 
isting in  an  atmosphere  around  the  sun,  which  absorb  the 
light  from  a  central  incandescent  nucleus,  each  metal  absorb- 
ing that  light  which  would  appear  as  a  bright  line  or  lines  in 
its  own  spectrum. 

By  comparing  the  position  of  the  bright  lines  in  the  spec- 
tra of  metals  with  that  of  the  dark  lines  in  the  solar  spec- 
trum, several  of  them  are  found  to  be  in  identically  the  same 
place  :  hence  it  is  inferred,  and  the  inference  seems  reason- 
able, that  the  metals  which  show  luminous  lines  in  their  spec- 
tra, identical  in  position  with  dark  lines  in  the  solar  spec- 
trum, exist  in  the  sun,  and  are  diffused  in  a  gaseous  state  in 
its  atmosphere.  It  does  not  seem  to  me  necessary  to  this 
conclusion  to  assume  that  the  sun  is  a  solid  mass  of  incandes- 
cent matter :  it  may  well  be  that  what  we  term  the  photo- 


HEAT.  63 

sphere  or  luminous  envelope  of  the  sun  has  surrounding  it  a 
more  diffuse  atmosphere  containing  vaporised  metals,  and 
that  the  mass  of  the  sun  itself  may  be  in  a  different  state, 
and  not  necessarily  at  an  incandescent  temperature ;  indeed, 
the  protuberances  and  red  light  seen  at  the  period  of  total 
eclipses  afford  some  evidence  of  an  atmosphere  exterior  to 
the  photosphere.  It  would,  however,  be  out  of  place  here  to 
speculate  on  these  subjects :  the  point  which  concerns  us  is 
the  analogies  of  heat  and  light,  which  these  discoveries  illus- 
trate. Kirchoff  has  carried  the  analogy  farther  by  showing 
that  a  plate  of  tourmaline  absorbs  the  polarised  ray  which 
when  heated  it  radiates.  Thus,  the  phenomena  of  light  are 
imitated  closely  by  those  of  radiant  heat ;  and  the  same  the- 
ory which  is  considered  most  plausibly  to  account  for  the 
phenomena  of  the  one,  will  necessarily  be  applied  to  the  other 
agent,  and  in  each  case  molecular  change  is  accompanied  by 
a  change  in  the  phenomenal  effects. 

In  certain  cases  heat  appears  to  become  partially  con- 
verted into  light,  by  changing  the  matter  affected  by  heat : 
thus  gas  may  be  heated  to  a  very  high  point  without  pro- 
ducing light,  or  producing  it  to  a  very  slight  degree  ;  but  the 
introduction  of  solid  matter — for  instance,  the  metal  platinum 
into  the  highly-heated  gas — instantly  exhibits  light.  Whether 
the  heat  is  converted  into  light,  or  whether  it  is  concentrated 
and  increased  in  intensity  by  the  solid  matter  so  as  to  become 
visible,  may  be  open  to  some  doubt :  the  fact  of  solid  matter, 
when  ignited  by  the  oxyhydrogen  jet  decomposing  water,  as 
will  be  presently  explained,  would  seem  to  indicate  that  the 
heat  was  rendered  more  intense  by  condensation  in  the  solid 
matter,  as  water  is  in  this  case  decomposed  by  a  heated  body, 
which  body  has  itself  been  heated  by  the  combining  elements 
of  water.  The  apparent  effect,  however,  of  the  introduction 
of  solid  incombustible  matter  into  heated  gas,  is  a  conversion 
of  heat  into  light. 

There  is  another  method  by  which  heat  would  probably 


64:  CORRELATION   OF   PHYSICAL   FORCES. 

be  made  to  produce  luminous  effects,  though  I  am  not  aware 
that  the  experiment  has  ever  been  made. 

If  we  concentrate  into  a  focus  by  a  large  lens  a  dim  light, 
we  increase  the  intensity  of  the  light.  Now  if  a  heated  body 
be  taken,  which,  to  the  unassisted  eye,  has  just  ceased  to  be 
visible,  it  seems  probable  that  by  collecting  and  condensing 
by  a  lens  the  different  rays  which  have  so  ceased  to  be  visi- 
ble, light  would  reappear  at  the  focus.  The  experiment  is, 
for  reasons  obvious  to  those  acquainted  with  optics,  a  difficult 
one,  and,  to  be  conclusive,  should  be  made  on  a  large  scale, 
and  with  a  very  perfect  lens  of  large  diameter  and  short  fo- 
cus. I  have  obtained  an  approximation  to  the  result  in  the 
following  manner : — In  a  dark  room  a  platinum  wire  is 
brought  just  to  the  point  of  visible  ignition  by  a  constant  vol- 
taic battery ;  it  is  then  viewed,  at  a  short  distance,  through 
an  opera-glass  of  large  aperture  applied  to  one  eye,  the  other 
being  kept  open.  The  wire  will  be  distinctly  visible  to  that 
eye  which  regards  it  through  the  opera-glass,  and  at  the  same 
time  totally  invisible  to  the  other  and  naked  eye.  It  may  be 
said  with  some  justice  that  such  experiments  prove  little  more 
than  the  fact  already  known,  viz.  that  by  increasing  the  in- 
tensity of  heat,  light  is  produced :  they  however  exhibit  this 
effect  in  a  more  striking  form,  as  bearing  on  the  relations  of 
heat  and  light. 

With  regard  to  chemical  affinity  and  magnetism,  perhaps 
the  only  method  by  which  in  strictness  the  force  of  heat  may 
be  said  to  produce  them  is  through  the  medium  of  electricity, 
the  thermo-electrical  current,  produced,  as  before  described, 
by  heating  dissimilar  metals,  being  capable  of  deflecting  the 
magnet,  of  magnetising  iron,  and  exhibiting  the  other  mag- 
netic effects,  and  also  of  forming  and  decomposing  chemical 
compounds,  and  this  in  proportion  to  the  progression  of  heat : 
this  has  not,  indeed,  as  yet  been  proved  to  bear  a  measurable 
quantitative  relation  to  the  other  forces  thus  produced  by  it, 
because  so  little  of  the  heat  is  utilised  or  converted  into  elec- 


HEAT.  65 

tricity,  much  being  dissipated,  without  change,  in  the  form  of 
heat. 

Heat,  however,  directly  affects  and  modifies  both  the  mag- 
net and  chemical  compounds  ;  the  union  of  certain  chemical 
substances  is  induced  by  heat,  as,  for  instance,  the  formation 
of  water  by  the  union  of  oxygen  and  hydrogen  gases :  in 
other  cases  this  union  is  facilitated  by  heat,  and  in  many  in- 
stances, as  in  ammonia  and  its  salts,  it  is  weakened  or  antag- 
onised. In  many  of  these  cases,  however,  the  force  of  heat 
seems  more  a  determining  than  a  producing  influence  ;  yet  to 
be  this,  it  must  have  an  immediate  relation  with  the  force 
whose  reaction  it  determines :  thus,  although  gunpowder, 
touched  with  an  ignited  wire,  subsequently  carries  on  its  own 
combustion  or  chemical  combination,  independently  of  the 
original  source  of  heat,  yet  the  chemical  affinities  of  the  first 
portion  touched  must  be  exalted  by,  and  at  the  cost  of,  the 
heat  of  the  wire  ;  for  to  disturb  even  an  unstable  equilibrium 
requires  a  force  in  direct  relation  with  those  which  maintain 
equilibrium. 

Since  the  first  edition  of  this  essay  was  published,  I  have 
communicated  to  the  Royal  Society  some  experiments  by 
which  an  important  exception  to  the  general  effect  of  heat  on 
chemical  affinity  is  removed,  and  the  results  of  which  induce 
a  hope  that  a  generalised  relation  will  ultimately  be  estab- 
lished between  heat,  chemical  affinity,  and  physical  attraction. 
I  find  that  if  a  substance  capable  of  supporting  an  intense 
heat,  and  incapable  of  being  acted  upon  by  water  or  either 
of  its  elements — such,  for  instance,  as  platinum,  or  iridium — 
be  raised  to  a  high  point  of  ignition  and  then  immersed  in 
water,  bubbles  of  permanent  gas  ascend  from  it,  which  on 
examination  are  found  to  consist  of  mixed  oxygen  and  hydro- 
gen in  the  proportions  in  which  they  form  water.  The  tem- 
perature at  which  this  is  effected  is,  according  to  Dr.  Robin- 
son, who  has  since  written  a  valuable  paper  on  the  subject, 
=  2386°.  Now,  when  mixed  oxygen  and  hydrogen  are  ex- 


DO  CORRELATION   OF   PHYSICAL   FORCES. 

posed  to  a  temperature  of  about  800°,  they  combine  and  form 
water ;  beat  therefore  appears  to  act  differently  upon  these 
elements  according  to  its  intensity,  in  one  case  producing 
composition,  in  the  other  decomposition.  No  satisfactory 
means  of  reconciling  this  apparent  anomaly  have  been  pointed 
out :  the  best  approximation  to  a  theory  which  I  can  frame  is 
by  assuming  that  the  constituent  molecules  of  water  are,  be- 
low a  certain  temperature,  in  a  state  of  stable  equilibrium ; 
that  the  molecules  of  mixed  or  oxyhydrogen  gas  are,  above  a 
certain  temperature,  also  in  a  state  of  stable  equilibrium,  but 
of  an  opposite  character ;  while  below  this  latter  tempera- 
ture the  molecules  of  mixed  gas  are  in  a  state  of  unstable 
equilibrium,  somewhat  similar  to  that  of  the  fulminates  or 
similar  bodies,  in  which  a  slight  derangement  subverts  the 
nicely-balanced  forces. 

If,  for  instance,  we  suppose  four  molecules,  A,  B,  C,  D, 
to  be  in  a  balanced  state  of  equilibrium  between  attracting 
and  repelling  forces,  the  application  of  a  repulsive  force  be- 
tween B  and  C,  though  it  may  still  farther  separate  B  and  C, 
will  approximate  B  to  A  and  C  to  D,  and  may  bring  them 
respectively  within  the  range  of  attractive  force ;  or,  sup- 
posing the  repulsive  force  to  be  in  the  centre  of  an  indefinite 
sphere  of  particles,  all  these,  excepting  those  immediately 
acted  on  by  the  force,  will  be  approximated,  and  having  from 
attraction  assumed  a  state  of  stable  equilibrium,  they  will  re- 
tain this,  because  the  repulsive  force  divided  by  the  mass  is 
not  capable  of  overcoming  it.  But  if  the  repulsive  force  be 
increased  in  quantity  and  of  sufficient  intensity,  then  the  at- 
tractive force  of  all  the  molecules  may  be  overcome,  and  de- 
composition ensue.  Thus,  water  or  steam  below  a  certain 
temperature,  and  mixed  gas  above  a  certain  temperature, 
may  be  supposed  to  be  in  a  state  of  stable  equilibrium,  whilst 
below  this  limiting  temperature,  the  equilibrium  of  oxyhy- 
drogen gas  is  unstable. 

This,  it  must  be  confessed,  is  but  a  crude  mode  of  explain- 


HEAT.  67 

ing  the  phenomena,  and  requires  the  assumption,  that  the 
particles  of  a  gas  exercise  an  attraction  for  each  other  as  do 
the  particles  of  a  solid,  though  different  in  degree,  perhaps  in 
kind.  Whether  this  be  so  or  not,  there  can  be  no  doubt  that 
both  .gases  and  solids  expand  or  contract  according  to  the  in- 
verse contraction  or  expansion  of  other  neighbouring  bodies, 
and  so  far  resemble  each  other  in  their  relations  to  heat  and 
cold.  The  extent  to  which  such  expansion  or  contraction 
can  be  carried,  seems  to  be  limited  only  by  the  correlative 
state  of  other  bodies  ;  these  again,  by  others,  and  so  on,  as 
far  as  we  may  judge,  throughout  the  universe. 

Adopting  the  explanation  above  given  of  the  decomposi- 
tion of  water  by  heat,  heat  would  have  the  same  relation  to 
chemical  affinity  as  it  has  to  physical  attraction ;  its  imme- 
diate tendency  is  antagonistic  to  both,  and  it  is  only  by  a  sec- 
ondary action  that  chemical  affinity  is  apparently  promoted 
by  heat.  This  view  would  explain  how  heat  may  promote 
changes  of  the  equilibrium  of  chemical  affinity  among  mixed 
compound  substances,  by  decomposing  certain  compounds  and 
separating  elementary  constituents  whose  affinity  is  greater, 
when  they  are  brought  within  the  sphere  of  attraction  for  the 
substance  with  which  they  are  mixed,  than  for  those  with 
which  they  were  originally  chemically  united :  thus  an  intense 
heat  being  applied  to  a  mixture  of  chlorine  and  the  vapour 
of  water,  occasions  the  production  of  muriatic  acid,  libera- 
ting oxygen. 

Carrying  out  this  view,  it  would  appear  that  a  sufficient 
intensity  of  heat  might  yield  indefinite  powers  of  decomposi- 
tion ;  and  there  seems  some  probability  of  bodies  now  sup- 
posed to  be  elementary,  being  decomposed  or  resolved  into 
further  elements  by  the  application  of  heat  of  sufficient  inten- 
sity ;  or,  reasoning  conversely,  it  may  fairly  be  anticipated 
that  bodies,  which  will  not  enter  into  combination  at  a  certain 
temperature,  will  enter  into  combination  if  their  temperature 
be  lowered,  and  that  thus  new  compounds  may  be  formed  by 


b»  CORRELATION   OF   PHYSICAL   FORCES. 

a  proper  disposition  of  their  constituents  when  expose*!  to  an 
extremely  low  temperature,  and  the  more  so  if  compression 
be  also  employed. 

In  considering  the  effect  of  heat  as  a  mechanical  force,  it 
would  be  expected,  d  priori,  and  independently  of  any  theory 
of  heat  which  may  be  adopted,  that  a  given  amount  of  heat 
acting  on  a  given  material  must  produce  a  given  amount  of 
motive  power ;  and  the  next  question  which  occurs  to  the 
mind  is,  whether  the  same  amount  of  heat  would  produce  the 
same  amount  of  mechanical  power,  whatever  be  the  material 
acted  on  or  affected  by  the  heat.  I  will  endeavour  to  reason 
this  out  on  the  view  of  heat  which  I  have  advocated.  Heat 
has  been  considered  in  this  essay  as  itself  motion  or  mechan- 
ical power,  and  quantity  of  heat  as  measured  by  motion. 
Thus,  if  by  a  given  contraction  of  a  body  (say  mercury)  air 
within  a  cylinder  having  a  moveable  piston  be  expanded,  the 
piston  moves,  and  in  this  case  the  expansion  or  motion  of  the 
material  (say  iron)  of  the  cylinder  itself  and  of  the  air  sur- 
rounding it  is  commonly  neglected.  As  the  air  dilates  it  be- 
comes colder  ;  in  other  words,  by  undergoing  expansion  itself, 
it  loses  its  power  of  making  neighbouring  bodies  expand ; 
but  if  the  piston  be  forcibly  kept  down,  the  expansive  power 
due  to  the  mercury  continues  to  communicate  itself  to  the 
iron  and  to  the  surrounding  air,  which  become  hotter  than 
they  would  if  the  piston  had  given  way. 

Now,  in  the  above  case,  if  the  air  be  confined  and  its 
volume  unchanged,  will  the  expansion  of  the  iron,  assuming 
that  it  can  be  utilised,  produce  an  exactly  equivalent  mechan- 
ical effect  to  that  which  the  expansion  of  the  air  would  pro- 
duce if  the  heat  be  entirely  confined  to  it  ? 

Assuming  that  (with  the  exception  of  bodies  which  ex- 
pand in  freezing,  where,  through  a  limited  range  of  tempera- 
ture, the  converse  effects  obtain)  whenever  a  body  is  com- 
pressed it  is  heated,  i.  e.  it  expands  neighbouring  substances  ; 
whenever  it  is  dilated  or  increased  in  volume  it  is  cooled,  i.  e. 


HEAT.  69 

it  contracts  neighbouring  substances — the  conclusion  ap- 
pears to  me  inevitable  that  the  mechanical  power  produced 
by  heat  will  be  definite,  or  the  same  for  a  given  amount  and 
intensity  of  heat,  whatever  be  the  substance  acted  on. 

Thus,  let  A  be  a  definite  source  of  heat,  say  a  pound  of 
mercury  at  the  temperature  of  400°  ;  let  B  be  another  equal 
and  similar  source  of  heat :  suppose  A  be  employed  to  raise 
a  piston  by  the  dilatation  of  air,  and  B  to  raise  another  pis- 
ton by  the  dilatation  of  the  vapour  of  water.  Imagine  the 
pistons  attached  to  a  beam,  so  that  they  oppose  each  other's 
action,  and  thus  represent  a  sort  of  calorific  balance.  If  A 
being  applied  to  air  could  conquer  B,  which  is  applied  to 
water,  it  would  depress  or  throw  back  the  piston  of  the  latter, 
and,  by  compressing  the  vapour,  occasion  an  increase  of 
temperature ;  this,  in  its  turn,  would  raise  the  temperature 
of  the  source  of  heat,  so  that  we  should  have  the  anomaly 
that  a  pound  of  mercury  at  400°  could  heat  another  pound 
of  mercury  at  400°  to  401°,  or  to  some  point  higher  than  its 
original  temperature,  and  this  without  any  adventitious  aid : 
it  will  be  obvious  that  this  is  impossible,  at  least  contradic- 
tory to  the  whole  range  of  our  experience. 

The  above  experiment  is  ideal,  and  stated  for  the  object 
of  giving  a  more  precise  form  to  the  reasoning ;  to  bring  the 
idea  more  prominently  into  relief,  all  statements  as  to  quan- 
tities, specific  heats,  &c.,  so  as  to  yield  comparative  results 
for  given  materials,  are  omitted.  The  argument  may  be 
thus  stated  in  another  form,  viz.  that  by  no  mechanical  appli- 
ance or  difference  of  material  acted  on  can  a  given  source 
of  heat  be  made  to  produce  more  heat  than  it  originally 
possessed ;  and  that,  if  all  be  converted  into  mechanical 
power,  an  excess  cannot  be  supposed,  for  that  could  be  con- 
verted into  a  surplus  of  heat,  and  be  a  creation  of  force  ;  and 
a  deficit  cannot  be  supposed,  for  that  would  be  annihilation 
of  force.  I  cannot,  however,  see  how  the  theoretical  concep- 
tion could  be  verified  by  experiment ;  the  enormous  weights 


70  CORRELATION   OF  PHYSICAL  FORCES. 

and  the  complex  mechanical  contrivances  requisite  to  give 
the  measure  of  power  yielded  by  matter  in  its  less  dilatable 
forms,  would  be  far  beyond  our  present  experimental  re- 
ources.  It  would  also  be  difficult  to  prevent  the  interference 
of  molecular  attractions,  inertia,  &c.,  the  overcoming  of 
which  expends  a  part  of  the  mechanical  power  generated, 
but  which  could  hardly  be  made  to  appear  in  the  result. 
We  could  not,  for  instance,  practically  realise  the  above  con- 
ception by  the  construction  of  a  machine  which  should  act  by 
the  expansion  and  contraction  of  a  bar  of  iron,  and  produce  a 
power  equal  to  that  of  a  steam  engine,  supplied  with  an  equal 
quantity  of  heat. 

Carnot,  who  wrote  in  1824  an  essay  on  the  motive  power 
of  heat,  regarded  the  mechanical  power  produced  by  heat  as 
resulting  from  a  transfer  of  heat  from  one  point  to  another, 
without  any  ultimate  loss  of  heat.  Thus,  in  the  action  of  an 
ordinary  steam  engine,  the  heat  from  the  furnace  having  ex- 
panded the  water  of  the  boiler  and  raised  the  piston,  a 
mechanical  motion  is  produced  ;  but  this  cannot  be  continued 
without  the  removal  of  the  heat,  or  the  contraction  of  the  ex- 
panded water.  This  is  done  by  the  condenser,  and  the  piston 
descends.  But  then  we  have  apparently  transferred  the  heat 
from  the  furnace  to  the  condenser,  and  in  the  transfer  effected 
mechanical  motion. 

Should  the  mechanical  motion  produced  by  heat  be  con- 
sidered as  the  effect  of  a  simple  transference  of  heat  from  one 
point  to  another,  or  as  the  result  of  a  conversion  of  heat  into 
the  mechanical  force  of  which  this  motion  is  the  result  ?  This 
question  leads  to  the  following :  does  the  heat  which  generates 
the  mechanical  power  return  to  the  thermal  machine  as  heat, 
or  is  it  conveyed  away  by  the  work  performed  ? 

If  a  definite  quantity  of  air  be  heated  it  is  expanded,  and 
by  its  expansion  it  cools  or  loses  some  of  its  power  of  com- 
municating heat  to  neighbouring  bodies.  That  which  we 
should  have  called  heat  if  the  expansion  of  the  air  had  been 


HEAT.  71 

prevented,  we  call  mechanical  effect,  or  may  view  as  converted 
into  mechanical  effect  ceasing  to  be  heat ;  but,  throwing  out 
of  the  question  nervous  sensation,  this  expansion  or  mechani- 
cal effect  is  all  the  evidence  we  have  of  heat,  for  if  the  air  is 
allowed  to  expand  freely,  this  expansion  becomes  the  index 
of  the  heat ;  if  the  air  be  confined,  the  expansion  of  the 
matter  of  the  vessel  confining  it,  or  of  the  mercury  of  a  ther- 
mometer in  contact  with  it,  &c.,  are  the  indices  of  the  heat. 

If,  again,  the  air  which  has  been  expanded  be,  by  mechani- 
cal pressure  or  by  other  means,  restored  to  its  original  bulk, 
it  is  capable  of  heating  or  expanding  other  substances  to  a 
degree  to  which  it  would  not  be  equal,  if  it  had  remained  in 
its  expanded  state.  To  produce  continuous  motion,  or  the 
up  and  down  stroke  of  a  piston,  we  must  heat  and  cool,  just 
as  with  a  magnetic  machine  we  must  magnetise  and  demagne- 
tise in  order  to  produce  a  continuous  mechanical  effect ;  and 
although,  from  the  impossibility  of  insulating  heat,  some  heat 
is  apparently  lost  in  the  process,  the  result  may  be  said  to  be" 
effected  by  the  transfer  of  heat  from  the  hot  to  the  cold  body, 
from  the  furnace  to  the  condenser.  But  we  may  equally  well 
say  that  the  heat  has  been  converted  into  mechanical  force, 
and  the  mechanical  force  back  into  heat ;  the  effects  are 
always  correlative,  as  are  the  mechanical  effects  of  an  air 
pump,  with  which,  as  we  dilate  the  air  on  one  side,  we  con 
dense  it  on  the  other ;  and  as  we  cannot  dilate  without  the 
reciprocal  condensation,  so  we  cannot  heat  without  the  recip- 
rocal cooling,  or  vice  versa. 

Hitherto  the  resistances  of  the  piston  or  of  any  superim- 
posed weight  have  been  thrown  out  of  consideration,  or,  what 
amounts  to  the  same  thing,  it  has  been  assumed  that  the 
weight  raised  by  the  piston  has  descended  with  it.  The  heat 
has  not  merely  been  employed  in  dilating  the  air  or  vapour, 
but  in  raising  the  piston  with  its  weight.  If,  as  the  vapour 
is  cooled,  the  weight  be  permitted  to  descend,  its  mechanical 
force  restores  the  heat  lost  by.  the  dilatation  ;  but  in  this  case 


72  CORRELATION   OF  PHYSICAL   FORCES. 

no  part  of  the  power  can  be  abstracted  so  as  to  be  employed 
for  any  practical  purpose :  this  question  then  follows,  what 
takes  place  with  regard  to  the  initial  heat,  if,  after  the  ascent 
of  the  piston,  the  weight  be  removed  so  as  not  to  help  the  pis- 
ton in  its  descent,  but  to  fall  upon  a  lever  or  produce  some 
extraneous  mechanical  effect  ? 

To  answer  this  question,  let  us  suppose  a  weight  to  rest 
on  a  piston  which  confines  air  at  a  definite  temperature,  say 
for  example  50°,  in  a  cylinder,  the  whole  being  assumed 
to  be  absolutely  non-conducting  for  heat.  A  part  of  the  heat 
of  this  confined  air  will  be  due  to  the  pressure,  since,  as 
we  have  seen,  compression  of  an  elastic  fluid  produces 
heat. 

Suppose,  now,  the  confined  air  to  be  heated  to  70°,  the 
piston  with  its  superincumbent  weight  will  ascend,  and  the 
temperature,  in  consequence  of  the  dilatation  of  the  air,  will 
be  somewhat  lowered,  say  to  69°  (we  will  assume,  for  the 
sake  of  simplicity,  that  the  heat  engendered  by  the  friction  of 
the  piston  compensates  the  force  lost  by  friction). 

The  piston  having  reached  its  maximum  of  elevation,  let 
a  cold  body  or  condenser  take  away  20°  from  the  temperature 
of  the  confined  air  ;  the  piston  will  now  descend,  and  by  the 
compression  which  the  weight  on  it  produces,  will  restore  the 
1°  lost  by  dilatation,  and  when  the  piston  reaches  its  original 
position  the  temperature  of  the  air  will  be  restored  to  50°. 
Suppose  this  experiment  repeated  up  to  the  rise  of  the  piston  ; 
but  when  the  piston  is  at  its  full  elevation,  and  the  cold  body 
applied,  let  the  weight  be  removed,  so  as  drop  upon  a  wheel, 
or  to  be  used  for  other  mechanical  purposes.  The  descend- 
ing piston  will  not  now  reach  its  original  point  without  more 
heat  being  abstracted ;  in  consequence  of  the  removal  of  the 
weight,  there  will  not  be  the  same  force  to  restore  the  1°,  and 
the  temperature  will  be  49°,  or  some  fraction  short  of  the 
original  50°.  If  this  were  otherwise,  then,  as  the  weight  in 
falling  may  be  made  to  produce  heat  by  friction,  we  should 


HEAT.  73 

have  more  heat  than  at  first,  or  a  creation  of  heat  out  of  noth- 
ing— in  other  words,  perpetual  motion. 

Let  us  now  assume  that  this  20°  supplied  in  the  first  in- 
stance was  yielded  by  a  body  at  90°,  of  such  size  and  material 
that  its  total  capacity  for  heat  is  equal  to  that  of  the  mass  of 
confined  air :  this  body  would  be  reduced  in  temperature  to 
70°,  in  other  words,  our  furnace  would  have  lost  20°  of  heat. 
Let  the  cold  body  of  the  same  size  and  material,  used  as  a 
condenser,  be  at  30°.  In  the  first  experiment,  the  body  at 
30°  would  bring  back  the  piston  to  its  original  point ;  but  in 
the  second  experiment,  or  that  where  the  weight  has  been 
removed,  the  body  at  30°  would  not  suffice  to  restore  the  pis- 
ton :  to  effect  this,  the  cold  body  or  condenser  must  be  at  a 
lower  temperature. 

The  question  in  Carnot's  theory,  which  is  not  experi- 
mentally resolved,  and  which  presents  extreme  experimental 
difficulty,  is  the  following:  Granted  that  a  piston  with  a 
superimposed  weight  be  raised  by  the  thermic  expansion  of 
confined  gas  or  vapour  below  it ;  if  the  elastic  medium  be 
restored  to  its  original  temperature  by  cooling,  the  weight  in 
depressing  the  piston  will  restore  that  portion  of  the  heat 
which  has  been  lost  by  the  expansion,  and  by  the  mechanical 
effect  consequent  thereon  ;  but  if  the  weight  be  removed  when 
at  its  maximum  of  elevation,  and  the  piston  be  brought  back 
to  its  starting  point  by  a  necessarily  cooler  body  than  could 
restore  it  if  the  weight  were  not  removed,  would  the  return  of 
the  piston  now  restore  the  heat  which  had  been  lost  by  the 
dilatation,  or,  in  other  words,  would  pulling  the  piston  down 
by  cold  restore  the  heat  equally  with  the  pressing  it  down  by 
mechanical  force?  The  argument  from  the  impossibility  of 
perpetual  motion  would  say  no,  for  if  all  the  heat  were 
restored,  the  mechanical  effect  produced  by  the  fall  of  the 
weight,  or  the  heating  effect  which  might  be  made  to 
result  from  this  mechanical  power,  would  be  got  from 
nothing. 

.       4 


74:  COEKELATION   OF   PHYSICAL   FOKCES. 

Then  follows  another  question,  viz.  whether,  where  an  ex- 
ternal or  derived  mechanical  effect  has  been  obtained,  would 
the  return  of  the  piston,  effected  without  the  weight  or  exter- 
nal force  to  assist  it,  but  solely  by  the  colder  body,  give  to 
this  latter  the  same  number  of  thermometric  degrees  as  had 
been  lost  by  the  hot  body  in  the  first  instance  ?  Suppose,  for 
instance,  the  cold  body  in  our  experiment  to  be  at  20°  instead 
of  30°,  would  this  body  gain  20°,  and  then  reach  the  tempera- 
ture of  40°  when  the  piston  is  brought  back,  or  would  its 
temperature  be  higher  or  lower  than  40°  ?  The  argument 
from  the  impossibility  of  perpetual  motion  does  not  apply 
here,  for  it  does  not  necessarily  follow  that  20°,  on  the  ther- 
mometric scale  from  20°  to  40°,  represents  an  equal  amount 
of  force  to  20°  on  the  scale  from  70°  to  90°,  and  therefore  it 
is  quite  conceivable  that  we  may  lose  20°  from  the  furnace, 
and  gain  20°  in  the  condenser,  and  yet  have  obtained  a  cer- 
tain amount  of  derived  mechanical  power.  It  will  also  follow, 
upon  a  consideration  of  the  above  imaginary  experiments, 
that  the  greater  the  mechanical  power  required,  the  greater 
should  be  the  difference  between  the  temperature  of  the 
furnace  and  that  of  the  condenser ;  but  the  exact  relation  in 
temperature  between  these,  for  a  given  mechanical  effect,  has 
not,  as  far  as  I  am  aware,  been  satisfactorily  established  by 
experiment,  though  it  has  been  shown  that  steam  at  high 
pressure  produces,  comparatively,  a  greater  mechanical 
effect  for  the  same  number  of  degrees  than  steam  at  low 
pressure. 

Carnot,  assuming  the  number  of  degrees  of  temperature 
to  be  restored,  but  at  a  lower  point  of  the  thermometric  scale, 
termed  this  the  fall  (chute)  of  caloric.  The  mechanical  effect 
of  heat,  on  this  view,  may  be  likened  to  that  of  a  series  of 
cascades  on  water-wheels.  The  highest  cascade  turns  a 
wheel,  and  produces  a  given  mechanical  effect ;  the  water 
which  has  produced  this  cannot  again  effect  it  at  the  same 
level  without  being  carried  back  to  its  original  elevation,  i.  e. 


HEAT.  75 

without  an  extra  force  being  employed  equivalent  to,  or 
rather  a  fraction  more  than  the  force  of  the  descending 
water ;  but  though  its  power  is  spent  with  reference  to  the 
first  wheel,  the  same  water  may,  by  falling  over  a  new 
precipice  upon  a  second  wheel,  again  reproduce  the  same 
mechanical  effect  (strictly  speaking,  rather  more,  for  it  has 
approximated  the  centre  of  gravity),  and  so  on,  until  no 
lower  fall  can  be  attained.  So  with  heat:  it  involves  no 
necessity  of  assuming  perpetual  motion  to  suppose  that,  after 
a  given  mechanical  effect,  produced  by  a  certain  loss  of  heat, 
the  number  of  degrees  lost  from  the  original  temperature 
may  be  restored  to  the  condenser,  but  at  a  lower  point  of  the 
thermometric  scale. 

If  work  has  been  done,  i.  e.  if  force  has  been  parted 
with,  the  original  temperature  itself  cannot  be  restored,  but 
there  is  no  a  priori  impossibility  in  the  same  number  of 
degrees  of  heat  as  have  been  converted  into  work  being  con- 
veyed to  a  condensing  body  so  cold  that,  when  it  receives  this 
heat,  it  will  still  be  below  the  original  temperature  to  which 
the  work-producing  heat  was  added. 

In  the  theory  of  the  steam-engine,  this  subject  possesses 
a  great  practical  interest.  Watt  supposed  that  a  given 
weight  of  water  required  the  same  quantity  of  what  is 
termed  total  heat  (that  is,  the  sensible  added  to  the  latent 
heat)  to  keep  it  in  the  state  of  vapour,  whatever  was  the 
pressure  to  which  it  was  subjected,  and,  consequently,  how- 
ever its  expansive  force  varied.  Clement  Desormes  was 
also  supposed  to  have  experimentally  verified  this  law.  If 
this  were  so,  vapour  raising  a  piston  with  a  weight  attached 
would  produce  mechanical  power ;  and  yet,  the  same  heat 
existing  as  at  first,  there  would  be  no  expenditure  of  the 
initial  force  ;  and  if  we  suppose  that  the  heat  in  the  condens- 
er was  the  real  representative  of  the  original  heat,  we 
should  get  perpetual  motion.  Southern  supposed  that  the 
latent  heat  was  constant,  and  that  the  heat  of  vapour  under 


76  COEEELATION   OF   PHYSICAL   FORCES. 

pressure  increased  as  the  sensible  heat.  M.  Despretz,  in 
1832,  made  some  experiments,  which  led  him  to  the  con- 
clusion that  the  increase  was  not  in  the  same  ratio  as  the 
sensible  heatr  but  that  yet  there  was  an  increase  ;  a  result 
confirmed  and  verified  with  great  accuracy  by  M.  Regnault, 
in  some  recent  and  elaborate  researches.  What  seems  to 
have  occasioned  the  error  in  Watt  and  Clement  Desormes' 
experiments  was,  the  idea  involved  in  the  term  latent  heat ; 
by  which,  supposing  the  phenomenon  of  the  disappearance  of 
sensible  heat  to  be  due  to  the  absorption  of  a  material  sub- 
stance, that  substance,  '  caloric/  was  thought  to  be  restored 
when  the  vapour  was  condensed  by  water,  even  though  the 
water  was  not  subjected  to  pressure  ;  but  to  estimate  the 
total  heat  of  vapour  under  pressure  the  vapour  should  be 
condensed  while  subjected  to  the  same  pressure  as  that  under 
which  it  is  generated,  as  was  done  in  M.  Despretz  and  M. 
Regnault's  experiments. 

M.  Seguin,  in  1839,  controverted  the  position  that  derived 
power  could  be  got  by  the  mere  transfer  of  heat,  and  by 
calculation  from  certain  known  data,  such  as  the  law  of  Mar- 
iotte,  viz.  that  the  elastic  force  of  gases  and  vapours  increas- 
ed directly  with  the  pressure  ;  and  assuming  that  for  vapour 
between  100°  and  150°  centigrade,  each  degree  of  elevation 
of  temperature  was  produced  by  a  thermal  unit,  he  deduced 
the  equivalent  of  mechanical  work  capable  of  being  perform- 
ed by  a  given  decrement  of  heat ;  and  thus  concluded  that, 
for  ordinary  pressures,  about  one  gramme  of  water  losing 
one  degree  centigrade  would  produce  a  force  capable  of  rais- 
ing a  weight  of  500  grammes  through  a  space  of  one  metre : 
this  estimate  is  a  little  beyond  that  given  by  the  converse  ex- 
periments of  Mr.  Joule,  already  stated,  in  which  the  heat 
produced  by  a  given  amount  of  mechanical  action  is  estimat- 
ed. I  am  not  aware  that  the  amount  of  mechanical  work 
which  is  produced  by  a  given  quantity  of  heat  has  been  di- 
rectly established  by  experiment,  though  some  approximative 


HEAT.  77 

results  in  particular  cases  have  been  given.  Theoretically  it 
should  be  the  same — that  is  to  say,  if  a  fall  of  772  Ibs. 
through  a  space  of  one  foot  will  raise  the  temperature  of  1  Ib. 
of  water  through  one  degree  of  Fahrenheit,  then  the  fall  in 
the  temperature  of  1  Ib.  of  water  through  one  degree  of  Fah- 
renheit should  be  able  to  raise  772  Ibs.  through  a  space  of 
one  foot.  The  calculations  of  M.  Seguin  are  not  far  from 
this,  but  since  the  elaborate  experiments  of  M.  Regnault  he 
has  expressed  some  doubt  of  the  correctness  of  his  former 
estimate,  as  by  these  experiments  it  appears  that,  within  cer- 
tain limits,  for  elevating  the  temperature  of  compressed  va- 
pour by  one  degree,  no  more  than  about  three-tenths  of  a  de- 
gree of  total  heat  is  required  ;  consequently,  the  equivalent 
multiplied  in  this  ratio  would  be  1,666  grammes,  instead  of 
500.  Other  investigators  have  given  numbers  more  or  less 
discordant ;  so  that,  without  giving  any  opinion  on  their  dif- 
ferent results,  this  question  may  be  considered  at  present  far 
from  settled.  M.  Regnault  himself  does  not  give  the  law  by 
which  the  ratio  of  heat  varies  with  reference  to  the  pressure, 
and  is  still  believed  to  be  engaged  in  researches  on  the  sub- 
ject— one  involving  questions  of  which  experiments  on  the 
mechanical  effects  of  elastic  fluids  seem  to  offer  the  most  pro- 
mising means  of  solution. 

I  have  endeavoured  to  give  a  proof  (by  showing  the 
anomaly  to  which  the  contrary  conclusion  would  lead)  that, 
whatever  amount  of  mechanical  power  is  produced  by  one 
mode  of  application  of  heat,  the  same  should,  in  theory,  be 
equally  produced  by  any  other  mode.  But  in  practice  the 
difference  is  immense  ;  and  therefore  it  becomes  a  question 
of  great  interest  practically  to  ascertain  what  is  the  most 
convenient  medium  on  which  to  apply  the  heat  employed,  and 
the  best  machinery  for  economising  it.  One  great  problem 
to  be  solved  is  the  saving  of  the  heat  which  the  steam  in  or- 
dinary engines,  after  having  done  its  work,  carries  into  the 
condenser,  or,  in  the  high-pressure  engine,  into  the  air.  It 


78  COKKELATION   OF   PHYSICAL   FORCES. 

is  argued  you  have  a  large  amount  of  fuel  consumed  to  raise 
water  to  the  boiling  point,  at  which  its  efficiency  as  a  motive 
agent  commences.  After  it  has  done  a  small  portion  of  work, 
and  while  it  still  retains  a  very  large  portion  of  the  heat  ori- 
ginally communicated  to  it,  you  reject  it,  and  have  to  start 
again  with  a  fresh  portion  of  steam  which  has  similarly  ex- 
hausted fuel — in  other  words,  you  throw  away  all,  and  more 
than  all  the  heat  which  has  been  employed  in  raising  the 
water  to  the  boiling  point.  Various  plans  have  been  devised 
to  remedy  this.  Using  again  the  warm  water  of  the  conden- 
ser to  feed  the  boiler  regains  a  part,  but  a  very  small  part,  of 
the  heat.  Employing  the  steam  first  for  a  high  pressure,  and 
then  before  its  rejection  or  condensation  using  it  for  a  low 
pressure,  cylinder,  is  a  second  mode  ;  a  third  is  to  use  the 
steam,  after  it  has  done  its  work  on  the  piston,  as  a  source  of 
heat  or  second  furnace,  to  boil  ether,  or  some  liquid  which 
evaporates  at  a  lower  temperature  than  water.  These  plans 
have  certain  advantages  ;  but  the  complexity  of  apparatus, 
the  danger  from  combustion  of  ether,  and  other  reasons, 
have  hitherto  precluded  their  general  adoption.  Under  the 
term  regenerating  engine  various  ingenious  combinations 
have  lately  been  suggested,  and  some  experimental  engines 
tried,  with  what  success  it  is  perhaps  too  early  at  present  to 
pronounce  an  opinion.  The  fundamental  notion  on  which 
this  class  of  engine  is  based  is  that  the  vapour  or  air,  when 
it  has  performed  a  certain  amount  of  work,  as  by  raising  a 
piston,  should,  instead  of  being  condensed  or  blown  off,  be 
retained  and  again  heated  to  its  original  high  temperature, 
and  then  used  de  novo ;  or  that  it  should  impart  its  heat  to 
some  other  substance,  and  the  latter  in  turn  impart  it  to  the 
fresh  vapour  about  to  act.  The  latter  plan  has  been  proposed 
by  Mr.  Ericsson :  he  passes  the  air  which  has  done  its 
work  through  layers  of  wire  gauze,  which  are  heated  by  the 
rejected  air,  and  through  which  the  next  charge  of  air  is 
made  to  pass.  M.  Seguin  and  Mr.  Siemens  have  construct- 


HEAT.  79 

ed  machines  upon  the  former  principle,  which  are  said  to 
have  given  good  experimental  results.  There  is,  however, 
a  theoretical  difficulty  in  all  these,  not  affecting  their  capabil- 
ity of  acting,  but  affecting  the  question  of  economy,  which  it 
does  not  seem  easy  to  escape  from.  Whether  the  heated  air 
or  vapour  be  retained,  or  whether  it  yield  its  heat  to  a  metal- 
lic or  other  substance,  this  heat  must  exercise  its  usual  repul- 
sive force,  and  this  must  re-act  either  against  the  returning 
piston  or  against  the  incoming  vapour,  and  require  a  greater 
pressure  in  that  to  neutralise  it.  Yapour  raising  a  piston  and 
producing  mechanical  force  effects  this  with  decreasing  power 
in  proportion  as  the  piston  is  moved.  At  a  certain  point  the 
piston  is  arrested,  or  the  stroke,  as  it  is  termed,  is  completed, 
but  there  is  still  compressed  vapour  in  the  cylinder  capable 
of  doing  work,  but  so  little  that  it  is,  and  must  in  practice 
be  neglected ;  if  this  compressed  vapour  be  retained,  the  pis- 
ton cannot  be  depressed  without  an  extra  force  capable  of 
over  coming  the  resistance  of  this,  so  to  speak,  semi-compress- 
ed vapour,  in  addition  to  that  which  is  requisite  to  produce  the 
normal  work  of  the  machine  ;  and  in  whatever  way  the  resi- 
dual force  be  retained,  it  must  either  be  antagonised  at  a  loss 
of  power  for  the  initial  force,  or  at  most  can  only  yield  the 
more  feeble  power  which  it  would  have  originally  given  if  it 
had  been  allowed  to  act  for  a  longer  stroke  on  the  piston.  It 
may  be  that  a  portion  of  this  residual  force  may  be  econo- 
mised ;  indeed,  this  is  done  when  the  boiler  is  charged  with 
warm  water  from  the  condenser,  instead  of  with  cold  water  ; 
but  some,  indeed  a  notable  loss,  seems  inevitable. 

Without  farther  discussing  the  various  inventions  and  the- 
ories on  this  subject,  which  are  daily  receiving  increased  de- 
velopment, it  may  be  well  to  point  out  how  far  nature  dis- 
tances art  in  its  present  state.  According  to  some  careful  es- 
timates, the  most  economical  of  our  furnaces  consume  from 
ten  to  twenty  times  as  much  fuel  to  produce  the  same  quantity 
of  heat  as  an  animal  produces ;  and  Matteucci  found  that. 


80  CORRELATION   OF   PHYSICAL    FOKCE8. 

from  a  given  consumption  of  zinc  in  a  voltaic  battery,  a  far 
greater  mechanical  effect  could  be  produced  by  making  it  act 
on  the  limbs  of  a  recently-killed  frog,  notwithstanding  the 
manifold  defects  of  such  an  arrangement  and  its  inferiority 
to  the  action  of  the  living  animal,  than  when  the  same  bat- 
tery was  made  to  produce  mechanical  power,  by  acting  on  an 
electro-magnetic  or  other  artificial  motor  apparatus.  The  ratio 
in  his  experiments  was  nearly  six  to  one.  Thus  in  all  our  arti- 
ficial combinations  we  can  but  apply  natural  forces,  and  with 
far  inferior  mechanism  to  that  which  is  perceptible  in  the 
economy  of  nature. 

Nature  is  made  better  by  no  mean ; 
But  nature  makes  that  mean ;  so  o'er  that  art, 
Which  you  say  adds  to  nature,  is  an  art 
That  nature  makes. 

A  speculation  has  been  thrown  out  by  Mr.  Thompson, 
that,  as  a  certain  amount  of  heat  results  from  mechanical  ac- 
tion, chemical  action,  &c.,  and  this  heat  is  radiated  into  space, 
there  must  be  a  gradual  diminution  of  temperature  for  the 
earth,  by  which  expenditure,  however  slow,  being  continuous, 
it  would  ultimately  be  cooled  to  a  degree  incompatible  with 
the  existence  of  animal  and  vegetable  life — in  short,  that  the 
earth  and  the  planets  of  our  system  are  parting  with  more 
heat  than  they  receive,  and  are  therefore  progressively  cool- 
ing. Geological  researches  support  to  some  extent  this  view, 
as  they  show  that  the  climate  of  many  portions  of  the  terres- 
trial surface  was  at  remote  periods  hotter  than  at  the  present 
time  :  the  animals  whose  fossilised  remains  are  found  in  an- 
cient strata  have  their  organism  adapted  to  what  we  should 
now  term  a  hot  climate.  There  are,  however,  so  many  cir- 
cumstances of  difficulty  attending  cosmical  speculations, 
that  but  little  reliance  can  be  placed  upon  the  most  profound. 
"We  know  not  the  original  source  of  terrestrial  heat ;  still 
less  that  of  the  solar  heat ;  we  know  not  whether  or  not  sys- 


HEAT.  81 

terns  of  planets  may  be  so  constituted  as  to  communicate 
forces,  inter  se,  so  that  forces  which  have  hitherto  escaped 
detection  may  be  in  a  continuous  or  recurring  state  of  inter- 
change. 

The  movements  produced  by  mutual  gravitation  may  be 
the  means  of  calling  into  existence  molecular  forces  within 
the  substances  of  the  planets  themselves.  As  neither  from 
observation,  nor  from  deduction,  can  we  fix  or  conjecture  any 
boundary  to  the  universe  of  stellar  orbs,  as  each  advance  in 
telescopic  power  gives  us  a  new  shell,  so  to  speak,  of 
stars,  we  may  regard  our  globe,  in  the  limit,  as  surrounded  by 
a  sphere  of  matter  radiating  heat,  light,  and  possibly  other, 
forces. 

Such  stellar  radiations  would  not,  from  the  evidence  we 
have  at  present,  appear  sufficient  to  supply  the  loss  of  heat 
by  terrestrial  radiations  ;  but  it  is  quite  conceivable  that  the 
whole  solar  system  may  pass  through  portions  of  space  hav- 
ing different  temperatures,  as  was  suggested,  I  believe,  by 
Poisson ;  that  as  we  have  a  terrestrial  summer  and  winter, 
so  there  may  be  a  solar  or  systematic  summer  and  winter,  in 
which  case  the  heat  lost  during  the  latter  period  might  be  re- 
stored during  the  former.  The  amount  of  the  radiations  of 
the  celestial  bodies  may  again,  from  changes  in  their  positions, 
vary  through  epochs  which  are  of  enormous  duration  as  re- 
gards the  existence  of  the  human  species. 

The  views  of  Mr.  Thompson  differ  from  those  of  Laplace, 
recently  enforced  by  M.  Babinet,  which  suppose  the  planets 
to  have  been  formed  by  a  gradual  condensation  of  nebulous 
matter.  A  modification  of  this  view  might,  perhaps,  be  sug- 
gested, viz.  that  worlds  or  systems,  instead  of  being  created 
as  wholes  at  definite  periods,  are  gradually  changing  by  at- 
mospheric additions  or  subtractions,  or  by  accretions  or  dim- 
inutions arising  from  nebulous  substance  or  from  meteoric 
bodies,  so  that  no  star  or  planet  could  at  any  time  be  said  to 
be  created  or  destroyed,  or  to  be  in  a  state  of  absolute  stabil- 
4* 


82  CORRELATION   OF   PHYSICAL   FORCES. 

ity,  but  that  some  may  be  increasing,  others  dwindling  away, 
and  so  throughout  the  universe,  in  the  past  as  in  the  future. 
When,  however,  questions  relating  to  cosmogony,  or  to  the 
beginning  or  end  of  worlds,  are  contemplated  from  a  physi- 
cal point  of  view,  the  period  of  time  over  which  our  experi- 
ence, in  its  most  enlarged  sense,  extends,  is  so  indefinitely 
minute  with  reference  to  that  which  must  be  required  for  any 
notable  change,  even  in  oar  own  planet,  that  a  variety  of  the- 
ories may  be  framed  equally  incapable  of  proof  or  of  dis- 
proof. We  have  no  means  of  ascertaining  whether  many 
changes,  which  endure  in  the  same  direction  for  a  term  be- 
yond the  range  of  human  experience,  are  really  continuous  or 
only  secular  variations,  which  may  be  compensated  for  at 
periods  far  beyond  our  ken,  so  that  in  such  cases  the  ques- 
tion of  comparative  stability  or  change  can  at  best  be  only 
answered  as  to  a  term  which,  though  enormous  with  refer- 
ference  to  our  computations,  sinks  into  nothing  with  reference 
to  cosmical  time,  if  cosmical  time  be  not  eternity.  Subjects 
such  as  these,  though  of  a  kind  on  which  the  mind  delights 
to  speculate,  appear,  with  reference  to  any  hope  of  attaining 
reliable  knowledge,  far  beyond  the  reach  of  any  present  or 
immediately  prospective  capacity  of  man. 


IV.   ELECTKICITY. 


is  that  affection  of  matter  or  mode  of 
force  which  most  distinctly  and  beautifully  relates  other 
modes  of  force,  and  exhibits,  to  a  great  extent  in  a  quantita- 
tive form,  its  own  relation  with  them,  and  their  reciprocal 
relations  with  it  and  with  each  other.  From  the  manner 
in  which  the  peculiar  force  called  electricity  is  seemingly 
transmitted  through  certain  bodies,  such  as  metallic  wires, 
the  term  current  is  commonly  used  to  denote  its  apparent 
progress.  It  is  very  difficult  to  present  to  the  mind  any 
theory  which  will  give  a  definite  conception  of  its  modus 
agendi:  the  early  theories  regard  its  phenomena  as  produced 
either  by  a  single  fluid  idio-repulsive,  but  attractive  of  all 
matter,  or  else  as  produced  by  two  fluids,  each  idio-repulsive 
but  attractive  of  the  other.  No  substantive  theory  has  been 
proposed  other  than  these  two  ;  but  although  this  is  the  case, 
I  think  I  shall  not  be  unsupported  by  many  who  have  atten- 
tively studied  electrical  phenomena,  in  viewing  them  as  re- 
sulting, not  from  the  action  of  a  fluid  or  fluids,  but  as  a  mole- 
cular polarisation  of  ordinary  matter,  or  as  matter  acting  by 
attraction  and  repulsion  in  a  definite  direction.  Thus,  the 
transmission  of  the  voltaic  current  in  liquids  is  viewed  by 
Grotthus  as  a  series  of  chemical  affinities  acting  in  a  definite 
direction  :  for  instance,  in  the  electrolysis  of  water,  i.  e.  its 
decomposition  when  placed  between  the  poles  or  electrodes 


84:  CORRELATION   OF   PHYSICAL    FORCES. 

of  a  voltaic  battery,  a  molecule  of  oxygen  is  supposed  to  be 
displaced  by  the  exalted  attraction  of  the  neighbouring  elec- 
trode ;  the  hydrogen  liberated  by  this  displacement  unites 
with  the  oxygen  of  the  contiguous  molecule  of  water  ;  this  in 
turn  liberates  its  hydrogen,  and  so  on ;  the  current  being 
nothing  else  than  this  molecular  transmission  of  chemical 
affinity. 

There  is  strong  reason  for  believing  that,  with  some  ex- 
ceptions, such  as  fused  metals,  liquids  do  not  conduct  elec- 
tricity without  undergoing  decomposition  ;  for  even  in  those 
extreme  cases  where  a  trifling  effect  of  conduction  is  appar- 
ently produced  without  the  usual  elimination  of  substances  at 
.the  electrodes,  the  latter  when  detached  from  the  circuit 
show,  by  the  counter-current  which  they  are  capable  of  pro- 
ducing when  immersed  in  a  fresh  liquid,  that  their  superficial 
state  has  been  changed,  doubtless  by  the  determination  to 
the  surfaces  of  minute  layers  of  substances  having  opposite 
chemical  characters.  The  question  whether  or  not  a  minute 
conduction  in  liquids  can  take  place  unaccompanied  by  chemi- 
cal action,  has  however  been  much  agitated,  and  may  be  re- 
garded as  inter  apices  of  the  science. 

Assuming  for  the  moment  electrolysis  to  be  the  only 
known  electrical  phenomenon,  electricity  would  appear  to  con- 
sist in  transmitted  chemical  action.  All  the  evidence  we 
have  is,  that  a  certain  affection  of  matter  or  chemical  change 
takes  place  at  certain  distant  points  of  space,  the  change  at 
one  point  having  a  definite  relation  to  the  change  at  the 
other,  and  being  capable  of  manifestation  at  any  intermediate 
points. 

If,  now,  the  electrical  effect  called  induction  be  examined, 
the  phenomena  will  be  found  equally  opposed  to  the  theory  of 
a  fluid,  and  consistent  with  that  of  molecular  polarisation. 
When  an  electrified  conductor  is  brought  near  another  which 
is  not  electrified,  the  latter  becomes  electrified  by  influence  or 
induction,  as  it  is  termed,  the  nearest  parts  of  each  of  these 


ELECTRICITY.  85 

two  bodies  exhibiting  states  of  electricity  of  the  contrary 
denominations.  Until  this  subject  was  investigated  by  Fara- 
day, the  intervening  non-conducting  body  or  dielectric  was 
supposed  to  be  purely  negative,  and  the  effect  was  attributed 
to  the  repulsion  at  a  distance  of  the  electrical  fluid.  Fara- 
day showed  that  these  effects  differed  greatly  according  to  the 
dielectric  that  was  interposed.  Thus  they  were  more  exalted 
with  sulphur  than  with  shellac  ;  more  with  shellac  than  with 
glass,  &c.  Matteucci,  though  differing  from  Faraday  as  to 
the  explanation  he  gave,  added  some  experiments  which  prove 
that  the  intervening  dielectric  is  molecularly  polarised.  Thus 
a  number  of  thin  plates  of  mica  are  superposed  like  a  pack 
of  cards  ;  metallic  plates  are  applied  to  the  outer  facings,  and 
one  of  them  electrified,  so  that  the  apparatus  is  charged  like 
a  Ley  den  phial.  Upon  separating  the  plates  with  insulating 
handles,  each  plate  is  separately  electrified,  one  side  of  it 
being  positive  and  the  other  negative,  showing  very  neatly 
and  decisively  a  polarisation  throughout  the  intervening  sub- 
stances by  the  effect  of  induction. 

Indeed,  chemical  action  or  electrolysis  may,  as  I  have 
shown,  be  transmitted  by  induction  across  a  dielectric  sub- 
stance, such  as  glass,  but  apparently  only  while  the  glass  is 
being  charged  with  electricity.  A  wire  passing  through  and 
hermetically  sealed  into  a  glass  tube,  a  short  portion  only  pro- 
jecting, is  made  to  dip  into  water  contained  in  a  Florence 
flask ;  the  flask  is  immersed  in  water  to  an  equal  depth  with 
that  within  it ;  the  wire  and  another  similar  wire  dipping 
into  the  outer  water  are  made  to  communicate  metallically 
with  the  powerful  electrical  machine  known  as  RhumkorPs 
coil ;  bubbles  of  gas  instantly  ascend  from  the  exposed  por- 
tions of  the  wires,  but  cease  after  a  certain  time,  and  are 
renewed  when,  after  an  interval  of  separation,  the  coil  is 
again  connected  with  the  wires. 

The  following  interesting  experiment  by  Mr.  Karsten 
goes  a  step  farther  in  corroboration  of  the  molecular  changes 


86  CORRELATION    OF   PHYSICAL   FORCES. 

consequent  upon  electrisation  :  A  coin  is  placed  on  a  pack  of 
thin  plates  of  glass,  and  then  electrified.  On  removing  the 
coin  and  breathing  on  the  glass  plate,  an  impression j)f  the 
coin  is  perceptible  ;  this  shows  a  certain  molecular  change  on 
the  surface  of  the  glass  opposed  to  the  plate,  or  of  the  vapours 
condensed  on  such  surface.  This  effect  might,  and  has  been 
interpreted  as  arising  from  a  film  of  greasy  deposit,  supposed 
to  exist  on  the  plate ;  the  impressions,  however,  have  been 
proved  to  penetrate  to  certain  depths  below  the  surface,  and 
not  to  be  removed  by  polishing. 

The  following  experiment,  however,  goes  farther :  On 
separating  carefully  the  glass  plates,  images  of  the  coin  can 
be  developed  on  each  of  the  surfaces,  showing  that  the  mole- 
cular change  has  been  transmitted  through  the  substance  of 
the  glass ;  and  we  may  thence  reasonably  suppose  that  a 
piece  of  glass,  or  other  dielectric  body,  if  it  could  be  split  up 
while  under  the  influence  of  electric  induction,  would  exhibit 
some  molecular  change  at  each  side  of  each  lamina,  however 
minutely  subdivided.  I  have  succeeded  in  farther  extending 
this  experiment,  and  in  permanently  fixing  the  images  thus 
produced  by  electricity.  Between  two  carefully-cleaned  glass 
plates  is  placed  a  w.ord  or  device  cut  out  of  paper  or  tinfoil ; 
sheets  of  tinfoil  a  little  smaller  than  the  glass  plates  are 
placed  on  the  outside  of  each  plate,  and  these  coatings  are 
brought  into  contact  with  the  terminals  of  Rhumkorf  s  coil. 
After  electrisation  for  a  few  seconds,  the  glasses  are  sepa- 
rated, and  their  interior  surfaces  exposed  to  the  vapour  of 
hydrofluoric  acid,  which  acts  chemically  on  glass ;  the  por- 
tions of  the  glass  not  protected  by  the  paper  device  are  cor- 
roded, while  those  so  protected  are  untouched  or  less 
affected  by  the  acid,  so  that  a  permanent  etching  is  thus 
produced,  which  nothing  but  disintegration  of  the  glass  will 
efface. 

Some  further  experiments  of  mine  on  this  subject  bring 
out  in  a  still  more  striking  manner  these  curious  molecular 


ELECTEICITY.  87 

changes.  One  of  the  plates  of  glass  having  been  electrified 
in  the  manner  just  mentioned,  is  coated,  on  the  side  impressed 
with  the  invisible  electrical  image,  with  a  film  of  iodised 
collodion  in  the  manner  usually  adopted  for  photographic 
purposes  ;  it  is  then  in  a  dark  room  immersed  in  a  solution 
of  nitrate  of  silver ;  then  exposed  to  diffuse  light  for  a  few 
seconds.  On  pouring  ever  the  collodion  the  usual  solution 
of  pyrogallic  acid,  the  invisible  electrical  image  is  brought 
out  as  a  dark  device  on  a  light  ground,  and  can  be  permanently 
fixed  by  hyposulphite  of  soda.  The  point  worthy  of  obser- 
vation in  this  experiment  is,  that  this  permanent  image  exists 
in  the  collodion  film,  which  can  be  stripped  off  the  glass,  dried, 
and  placed  on  any  other  surface,  so  that  the  molecular  change 
consequent  on  electrisation  has  communicated,  by  contact  or 
close  proximity,  a  change  to  the  film  of  collodion  corres- 
ponding in  form  with  that  on  the  glass,  but  being  undoubtedly 
of  a  chemical  nature.  Electricity  has,  moreover,  in  this  ex- 
periment so  modified  the  surface  of  glass,  that  it  can,  in  its 
turn,  modify  the  structure  of  another  substance  so  as  to  alter 
the  relation  of  the  latter  to  light.  It  would  require  a  curious 
complication  of  hypothetic  fluids  to  explain  this  ;  but  if  elec- 
tricity and  light  be  supposed  to  be  affections  of  ordinary  pon- 
derable matter,  the  difficulty  is  only  one  of  detail. 

If,  again,  we  examine  the.  electricity  of  the  atmosphere, 
when,  as  is  usually  the  case,  it  is  positive  with  respect  to  that 
of  the  earth,  we  find  that  each  successive  stratum  is  positive 
to  those  below  it  and  negative  to  those  above  it ;  and  the  con- 
verse is  the  case  when  the  electricity  of  the  atmosphere  is 
negative  with  respect  to  that  of  the  earth. 

If  another,  electrical  phenomenon  be  selected,  another  sort 
of  change  will  be  found  to  have  taken  place.  The  electric 
spark,  the  brush,  and  similar  phenomena,  the  old  theories 
regarded  as  actual  emanations  of  the  matter  or  fluid,  Elec- 
tricity ;  I  venture  to  regard  them  as  produced  by  an  emission 
of  the  material  itself  from  whence  they  issue,  and  a  molecular 


88  COKBELATION  OF   PHYSICAL   FOKCES. 

action  of  the  gas,  or  intermedium,  through  or  across  which 
they  are  transmitted. 

The  colour  of  the  electric  spark,  or  of  the  voltaic  arc  (i. 
e.  the  flame  which  plays  between  the  terminal  points  of  a 
powerful  voltaic  battery) ,  is  dependent  upon  the  substance  of 
the  metal,  subject  to  certain  modifications  of  the  intermedium  : 
thus,  the  electric  spark  or  arc  from  zinc  is  blue  ;  from  silver, 
green  ;  from  iron,  red  and  scintillating ;  precisely  the  colours 
afforded  by  these  metals  in  their  ordinary  combustion.  A 
portion  of  the  metal  is  also  found  to  be  actually  transmitted 
with  every  electric  or  voltaic  discharge  :  in  the  latter  case, 
indeed,  where  the  quantity  of  matter  acted  upon  is  greater 
than  in  the  former,  the  metallic  particles  emitted  by  the  elec- 
trodes or  terminals  can  be  readily  collected,  tested,  or  even 
weighed.  It  would  thus  appear  that  the  electrical  discharge 
arises,  at  least  in  part,  from  an  actual  repulsion  and  sever- 
ance of  the  electrified  matter  itself,  which  flies  off  at  the  points 
of  least  resistance. 

A  careful  examination  of  the  phenomena  attending  the 
electric  spark  or  the  voltaic  arc,  which  latter  is  the  electric 
disruptive  discharge  acting  on  greater  portions  of  matter, 
tends  to  modify  considerably  our  previous  idea  of  the  nature 
of  the  electric  force  as  a  producer  of  ignition  and  combustion. 
The  voltaic  arc  is  perhaps,  strictly  speaking,  neither  ignition 
nor  combustion.  It  is  not  simply  ignition  ;  because  the  mat- 
ter of  the  terminals  is  not  merely  brought  to  a  state  of  incan- 
descence, but  is  physically  separated  and  partially  transferred 
from  one  electrode  to  another,  much  of  it  being  dissipated  in 
a  vaporous  state.  It  is  not  combustion  ;  for  the  phenomena 
will  take  place  independently  of  atmospheric  air,  oxygen  gas, 
or  any  of  the  bodies  usually  called  supporters  of  combustion, 
combustion  being  in  fact  chemical  union  attended  with  heat 
and  light.  In  the  voltaic  arc  we  may  have  no  chemical  union  ; 
for  if  the  experiment  be  performed  in  an  exhausted  receiver,  or 
in  nitrogen,  the  substance  forming  the  electrodes  is  condensed, 


ELECTEICITY.  89 

and  precipitated  upon  the  interior  of  the  vessel  in,  chemically 
speaking,  an  unaltered  state.  Thus,  to  take  a  very  striking 
example,  if  the  voltaic  discharge  be  taken  between  zinc  ter- 
minals in  an  exhausted  receiver,  a  fine  black  powder  of  zinc 
is  deposited  on  the  sides  of  the  receiver ;  this  can  be  collect- 
ed, and  takes  fire  readily  in  the  air  by  being  touched  with  a 
match,  or  ignited  wire,  instantly  burning  into  white  oxide  of 
zinc.  To  an  ordinary  observer,  the  zinc  would  appear  to  be 
burned  twice — first  in  the  receiver,  where  the  phenomenon 
presents  all  the  appearance  of  combustion,  and  secondly  in 
the  real  combustion  in  air.  With  iron  the  experiment  is 
equally  instructive.  Iron  is  volatilised  by  the  voltaic  arc  in 
nitrogen  or  in  an  exhausted  receiver  ;  and  when  a  scarcely 
perceptible  film  has  lined  the  receiver,  this  is  washed  with  an 
acid,  which  then  gives,  with  ferrocyanide  of  potassium,  the  prus- 
sian-blue  precipitate.  In  this  case  we  readily  distil  iron,  a 
metal  by  ordinary  means  fusible  only  at  a  very  high  tempera- 
ture. 

Another  strong  evidence  that  the  voltaic  discharge  con- 
sists of  the  material  itself  of  which  the  terminals  are  compos- 
ed, is  the  peculiar  rotation  which  is  observed  in  the  light 
when  iron  is  employed,  the  magnetic  character  of  this  metal 
causing  its  molecules  to  rotate  by  the  influence  of  the  voltaic 
current. 

If  we  increase  the  number  of  reduplications  in  a  voltaic 
series,  we  increase  the  length  of  the  arc,  and  also  increase  its 
intensity  or  power  of  overcoming  resistance.  With  a  battery 
consisting  of  a  limited  number,  say  100  reduplications,  the 
discharge  will  not  pass  from  one  terminal  to  the  other  with- 
out first  bringing  them  into  contact,  but  if  we  increase  the 
number  of  cells  to  400  or  500,  the  discharge  will  pass  from 
one  terminal  to  the  other  before  they  are  brought  into  contact. 
The  difference  between  what  is  called  Franklinic  electricity,  or 
that  produced  by  an  ordinary  electrical  machine,  and  voltaic  elec- 
tricity, or  that  produced  by  the  ordinary  voltaic  battery,  is  that 


90  CORRELATION   OF   PHYSICAL    FORCES. 

the  former  is  of  much  greater  intensity  than  the  latter,  or  has  a 
greater  power  of  overcoming  resistance,  but  acts  upon  a  much 
smaller  quantity  of  matter.  If,  then,  a  voltaic  battery  be 
formed  with  a  view  to  increase  the  intensity  and  lessen  the 
quantity,  the  character  of  the  electrical  phenomena  approxi- 
mate those  of  the  electrical  machine.  In  order  to  effect  this, 
the  sizes  of  the  plates  of  the  battery  and  thence  the  quantity 
of  matter  acted  on  in  each  cell,  must  be  reduced,  but  the 
number  of  reduplications  increased.  Thus  if  in  a  battery  of  100 
pairs  of  plates  each  plate  be  divided,  and  the  battery  be  arranged 
so  as  to  form  200  pairs,  each  being  half  the  original  size,  the 
quantitative  effects  are  diminished,  and  the  effects  of  intensi- 
ty increased.  By  carrying  on  this  sub-division,  diminishing 
the  sizes  and  increasing  the  number,  as  is  the  case  in  the  vol- 
taic piles  of  Deluc  and  Zamboni,  effects  are  ultimately  pro- 
duced similar  to  those  of  Franklinic  electricity,  and  we  thus 
gradually  pass  from  the  voltaic  arc  to  the  spark  or  electric 
discharge. 

This  discharge,  as  I  have  already  stated,  has  a  colour  de- 
pending in  part  upon  the  nature  of  the  terminals  employed. 
If  these  terminals  be  highly  polished,  a  spot  will  be  observed, 
even  in  the  case  of  a  small  electric  spark,  at  the  points  from 
which  the  discharge  emanates.  The  matter  of  the  terminals 
is  itself  affected ;  and  a  transmission  of  this  matter  across 
the  intervening  space  is  detected  by  the  deposition  of  minute 
quantities  of  the  metal  or  substance  composing  the  one,  upon  the 
other  terminal. 

If  the  gas  or  elastic  medium  between  the  terminals  be 
changed,  a  change  takes  place  in  the  length  or  colour  of  the 
discharge,  showing  an  affection  of  the  intervening  matter. 
If  the  gas  be  rarefied,  the  discharge  gradually  changes  with 
th£  degree  of  rarefaction,  from  a  spark  to  a  luminous  glow  or 
diffuse  light,  differing  in  colour  in  different  gases,  and  capable 
of  extending  to  a  much  greater  distance  than  when  it  takes 
place  in  air  of  the  ordinary  density.  Thus,  in  highly  attenu- 


ELECTRICITY.  91 

ated  air  a  discharge  may  be  made  to  pass  across  six  or  seven 
feet  of  space,  while  in  air  of  the  ordinary  density  it  would 
not  pass  across  an  inch.  An  observer  regarding  the  beauti- 
ful phenomena  exhibited  by  this  electric  discharge  in  attenua- 
ted gas,  which,  from  some  degree  of  similarity  in  appear- 
ance to  the  Aurora  Borealis,  has  been  called  the  electric  Au- 
rora, would  have  some  difficulty  in  believing  such  effects 
could  be  due  to  an  action  of  ordinary  matter.  The  amount 
of  gas  present  is  extremely  small ;  and  the  terminals,  to  a 
cursory  examination,  show  no  change  after  long  experiment- 
ing. It  is  therefore  not  to  be  wondered  at  that  the  first  ob- 
servers of  this  and  similar  phenomena,  regarded  electricity 
as  in  itself  something — as  a  specific  existence  or  fluid.  Even 
in  this  extreme  case,  however,  upon  a  more  careful  examination 
we  shall  find  that  a  change  does  take  place,  both  as  regards  the  gas 
and  as  regards  the  terminals.  Let  one  of  these  consist  of  a  high- 
ly-polished metal — a  silver  plate  is  one  of  the  best  materials  for 
the  purpose — and  let  the  discharges  in  attenuated  atmospheric 
air  take  place  from  a  point,  say  a  common  sewing  needle,  to  the 
surface  of  the  polished  silver  plate  ;  it  will  be  found  that  this 
is  gradually  changed  in  appearance  opposite  the  point — it  is  ox- 
idated, and  gradually  more  and  more  corroded  as  the  discharge 
is  continued. 

If  now  the  gas  be  changed,  and  highly-rarefied  hydrogen 
be  substituted  for  the  rarefied  air,  all  other  things  remaining 
the  same,  upon  passing  the  discharges  as  before  the  oxide 
will  be  cleared  off  the  plate,  and  the  polish  to  a  great  extent 
restored — not  entirely,  because  the  silver  has  been  disinte- 
grated by  the  oxidation — and  the  portion  which  has  been  af- 
fected by  the  discharge  will  present  a  somewhat  different  ap- 
pearance from  the  remainder  of  the  plate. 

A  question  will  probably  here  occur  to  the  reader  : — WJiat 
will  be  the  effect  if  there  be  not  an  oxidating  medium  pres- 
ent, and  the  experiment  be  first  performed  in  a  rarefied  gas, 
which  possesses  no  power  of  chemically  acting  on  the  plate  ? 


92  CORRELATION   OF   PHYSICAL   FORCES. 

In  this  case  there  will  still  be  a  molecular  change  or  disinte- 
gration of  the  plate  ;  the  portion  of  it  acted  on  by  the  dis- 
charge will  present  a  different  appearance  from  that  which  is 
beyond  its  reach,  and  a  whitish  film,  somewhat  similar  to 
that  seen  on  the  mercurialised  portions  of  a  daguerreotype, 
will  gradually  appear  on  the  portion  of  the  plate  affected  by 
the  discharge.  If  the  gas  be  a  compound,  as  carbonic  oxide, 
or  a  mixture ,  as  oxygen  and  hydrogen,  and  consequently  contain 
elements  capable  of  producing  oxidation  and  reduction,  then 
the  effect  upon  the  plate  will  depend  upon  whether  it  be  pos- 
itive or  negative  ;  in  the  former  case  it  will  be  oxidated,  in 
the  latter  the  oxide,  if  existing,  will  be  reduced.  This  effect 
will  also  take  place  in  atmospheric  air,  if  it  be  highly  rare- 
fied, and  can  hardly  be  explained  otherwise  than  by  a  mole- 
cular polarisation  of  the  compound  gas.  If,  again,  the  metal 
be  reduced  to  a  small  point,  and  be  of  such  material  that  the 
gas  cannot  act  chemically  upon  it,  it  can  yet  be  shown  to  be 
disintegrated  by  the  electric  spark.  Thus,  let  a  fine  plati- 
num wire  be  hermetically  sealed  in  a  glass  tube,  and  the  ex- 
tremity of  the  tube  and  the  wire  ground  to  a  flat  surface ,  so  as  to 
expose  a  section  only  of  the  wire  ;  after  taking  the  discharge 
from  this  for  some  time,  it  will  be  found  that  the  platinum 
wire  is  worn  away,  and  that  its  termination  is  sensibly  below 
the  level  of  the  glass.  If  the  discharges  from  such  a  plati- 
num wire  be  taken  in  gas  contained  in  a  narrow  tube,  a  cloud 
or  film  consisting  of  a  deposit  of  platinum  will  be  seen  on 
the  part  of  the  tube  surrounding  the  point. 

Another  curious  effect  which,  in  addition  to  the  above,  I 
have  detected  in  the  electrical  discharge  in  attenuated  media, 
is  that  when  passing  between  terminals  of  a  certain  form,  as 
from  a  wire  placed  at  right  angles  to  a  polished  plate,  the  dis- 
charge possesses  certain  phases  or  fits  of  an  alternate  character, 
so  that,  instead  of  impressing  an  uniform  mark  on  a  polished 
plate,  a  series  of  concentric  rings  is  formed. 

Priestley  observed  that,  after  the  discharge  of  a  Leyden 


ELECTRICITY.  93 

battery,  rings  consisting  of  fused  globules  of  metal  were 
formed  on  the  terminal  plates  ;  in  my  experiments  made  in 
attenuated  media,  alternate  rings  of  oxidation  and  deoxida- 
tion  are  formed.  Thus,  if  the  plate  be  polished,  coloured 
rings  of  oxide  will  alternate  with  rings  of  polished  or  unoxi- 
dated  surface  ;  and  if  the  plate  be  previously  coated  with  an 
uniform  film  of  oxide,  the  oxide  will  be  removed  in  concen- 
tric spaces,  and  increased  in  the  alternate  ones,  showing  a 
lateral  alternation  of  positive  and  negative  electricity,  or 
electricity  of  opposite  character  in  the  same  discharge. 

It  would  be  hasty  to  assert  that  in  no  case  can  the  electri- 
cal disruptive  discharge  take  place  without  the  terminals  be- 
ing affected.  I  have,  however,  seen  no  instance  of  such  are- 
suit  where  the  discharge  has  been  sufficiently  prolonged,  and 
the  terminals  in  such  a  state  as  could  be  expected  to  render 
manifest  slight  changes. 

The  next  question  which  would  occur  in  following  out  the 
enquiry  which  has  been  indicated,  would  probably  be,  What 
is  the  action  upon  the  gas  itself?  is  this  changed  in  any  man- 
ner? 

In  answer  to  this,  it  must  be  admitted  that,  in  the  present 
state  of  experimental  knowledge  on  this  subject,  certain 
gases  only  appear  to  leave  permanent  traces  of  their  having 
been  changed  by  the  discharge,  while  others,  if  affected  by 
it,  which,  as  will  be  presently  seen,  there  are  reasons  to  be- 
lieve they  are,  return  to  their  normal  state  immediately  after 
the  discharge. 

In  the  former  class  we  may  place  many  compound  gases, 
as  ammonia,  olefiant  gas,  protoxide  of  nitrogen,  deutoxide 
of  nitrogen,  and  others,  which  are  decomposed  by  the  action 
of  the  discharge.  Mixed  gases  are  also  chemically  combined : 
for  instance,  oxygen  and  hydrogen  unite  and  form  water ; 
common  air  gives  nitric  acid ;  chlorine  and  aqueous  vapour 
give  oxygen,  the  chlorine  uniting  with  the  hydrogen  of  the 
water. 


94:  CORRELATION   OF   PHYSICAL   FORCES. 

But,  further  than  this,  in  the  case  of  certain  elementary 
gases  a  permanent  change  is  effected  by  the  electrical  dis- 
charge. Thus,  oxygen  submitted  to  the  discharge  is  par- 
tially changed  into  the  substance  now  considered  to  be  an  al- 
lotropic  condition  of  oxygen  ;  and  there  is  reason  to  believe 
that  when  the  change  takes  place,  there  is  a  definite  polar 
condition  of  the  gas,  and  that  definite  portions  of  it  are  affected 
— that  in  a  certain  sense  one  portion  of  the  oxygen  bears 
temporarily  to  the  other  the  relation  which  hydrogen  ordina- 
rily does  to  oxygen. 

If  the  discharge  be  passed  through  the  vapour  of  phos- 
phorus in  the  vacuum  of  a  good  air-pump,  a  deposit  of  allotro- 
pic  phosphorus  soon  coats  the  interior  of  the  receiver,  show- 
ing an  analogous  change  to  that  produced  in  oxygen  ;  and  in 
this  case  a  series  of  transverse  bands  or  stratifications  appears 
in  the  discharge,  showing  a  most  striking  alteration  in  its 
physical  character,  dependent  on  the  medium  across  which  it  is 
transmitted.  These  effects  were  first  observed  by  me  in 
the  year  1852.  They  have  since  been  much  examined  by 
continental  philosophers,  and  much  extended  by  Mr.  Gassiot ; 
but  no  satisfactory  rationale  of  them  has  yet  been  given. 

There  are  many  gases  which  either  do  not  show  any  per- 
manent change,  or  (which  is  more  probably  the  case)  the 
changes  produced  in  them  by  the  electrical  discharge  have 
not  yet  been  detected.  Even  with  these  gases,  however,  the 
difference  of  colour,  of  length,  or  of  the  different  position  of 
a  certain  dark  space  or  spaces  which  appear  in  the  discharge, 
show  that  the  discharge  differs  for  different  media.  We  nev- 
er find  that  the  discharge  has  itself  added  to  or  subtracted 
from  the  total  weight  of  the  substances  acted  on :  we  find  no 
evidence  of  a  fluid  but  the  visible  phenomena  themselves ; 
and  those  we  may  account  for  by  the  change  which  takes 
place  in  the  matter  affected. 

I  have  here,  as  elsewhere,  used  words  of  common  accep- 
tation, such  as  '  matter  affected  by  the  discharge/  &c.,  though 


ELECTRICITY.  95 

upon  the  view  I  am  suggesting,  the  discharge  is  itself  this 
affection  of  matter :  and  the  writing  these  passages  affords, 
to  me  at  least,  a  striking  instance  of  how  much  ideas  are 
bound  up  in  words,  when,  to  express  a  view  differing  from 
the  received  one,  words  involving  the  received  one  are  neces- 
sarily used. 

Passing  now  to  the  effect  of  the  transmission  of  electri- 
city by  the  class  of  the  best  conducting  bodies,  such  as  the 
metals  and  carbon,  here,  though  we  cannot  at  present  give  the 
exact  character  of  the  motion  impressed  upon  the  particles, 
there  are  yet  many  experiments  which  show  that  a  change 
takes  place  in  such  substances  when  they  are  affected  by  elec- 
tricity. 

Let  discharges  from  a  Ley  den  jar  or  battery  be  passed 
through  a  platinum  wire,  too  thick  to  be  fused  by  the  dis- 
charges, and  free  from  constraint,  it  will  be  found  that  the 
wire  is  shortened  ;  it  has  undergone  a  molecular  change,  and 
apparently  been  acted  on  by  a  force  tranverse  to  its  length. 
If  the  discharges  be  continued,  it  gradually  gathers  up  in 
small  irregular  bends  or  convolutions.  So  with  voltaic  elec- 
tricity :  place  a  platinum  wire  in  a  trough  of  porcelain,  so 
that  when  fused  it  shall  retain  its  position  as  a  wire,  and  then 
ignite  it  by  a  voltaic  battery.  As  it  reaches  the  point  of  fu- 
sion it  will  snap  asunder,  showing  a  contraction  in  length,  and 
consequently  a  distension  or  increase  in  its  transverse  dimen- 
sions. Perform  the  same  experiment  with  a  lead  wire, 
which  can  be  more  readily  kept  in  a  state  of  fusion,  and  fol- 
low it,  as  it  contracts,  by  the  terminal  wires  of  the  battery  ; 
it  will  be  seen  to  gather  up  in  nodules,  which  press  on  each 
other  like  a  string  of  beads  of  a  soft  material  which  have 
been  longitudinally  compressed. 

As  we  increase  the  thickness  of  the  wires  in  these  exper- 
iments with  reference  to  the  electrical  force  employed,  we  les- 
sen the  perceptible  effect :  but  even  in  this  case  we  shall  be 
enabled  safely  to  infer  that  some  molecular  change  accompa- 


CORRELATION   OF   PHYSICAL   FORCES. 

nies  the  transmission  of  electricity :  the  wires  are  heated  in  a 
degree  decreasing  as  their  thickness  increases — but  by  in- 
creasing the  delicacy  of  our  tests  as  the  heating  effects  de- 
crease in  intensity,  we  may  indefinitely  detect  the  augmenta- 
tion of  temperature  accompanying  the  passage  of  electri- 
city— and  wherever  there  is  augmentation  of  temperature 
there  must  be  expansion  or  change  of  position  of  the  mole- 
cules. 

Again,  it  has  been  observed  that  wires  which  have  for  a 
long  time  transmitted  electricity,  such  as  those  which  have 
served  as  conductors  for  atmospheric  electricity,  have  their 
texture  changed,  and  are  rendered  brittle.  In  this  observa- 
tion, however,  though  made  by  a  skillful  electrician,  M.  Pel- 
tier, the  effects  of  exposure  to  the  atmosphere,  to  changes  of 
temperature,  &c.,  have  not  been  sufficiently  eliminated  to 
render  it  worthy  of  entire  confidence.  There  are,  however, 
other  experiments  which  show  that  the  elasticity  of  metals  is 
changed  by  the  passage  through  them  of  the  electric  current. 

Thus  M.  Wertheim  has,  from  an  elaborate  series  of  ex- 
periments, arrived  at  the  conclusion  that  there  is  a  temporary 
diminution  in  the  coefficient  of  elasticity  in  wires  while  they 
are  transmitting  the  electric  current,  which  is  independent  of 
the  heating  effect  of  the  current. 

M.  Dufour  has  made  a  considerable  number  of  experi- 
ments with  the  view  of  ascertaining  if  any  permanent  change 
in  metals  is  effected  by  electrisation.  He  arrives  at  the  cu- 
rious result  that  in  a  copper  wire  through  which  a  feeble  vol- 
taic current  has  passed  for  several  days,  a  notable  diminution 
in  tenacity  takes  place ;  while,  in  an  iron  wire,  the  tenacity 
is  increased ;  and  that  these  effects  were  more  perceptible 
when  the  wires  had  been  electrised  for  a  long  time  (nineteen 
days)  than  for  a  short  time  (four  days).  The  copper  wire 
was,  in  his  experiment,  not  perfectly  pure  ;  so  that  the  effect, 
or  a  portion  of  it,  might  be  due  to  the  state  of  alloy :  in  the 
case  of  iron,  the  magnetic  character  of  the  metal  would  prob- 


ELECTRICITY.  97 

ably  modify  the  effects,  and  might  account  for  the  opposite 
character  of  the  results  with  these  two  metals. 

Matteucci  has  made  experiments  on  the  conduction  of 
electricity  by  bismuth  in  directions  parallel  or  transverse  to 
the  planes  of  principal  cleavage,  and  he  finds  that  bismuth 
conducts  electricity  and  heat  better  in  the  direction  of  the 
cleavage  planes  than  in  that  transverse  to  them. 

Many  other  experiments  have  been  made  both  on  the  pro- 
duction of  thermo-electric  currents  by  two  portions  of  the 
same  crystalline  metal,  but  with  the  planes  of  crystallization 
arranged  in  different  directions  relatively  to  each  other,  and 
also  on  the  differences  in  conduction  of  heat  and  electricity 
according  to  the  direction  in  which  they  are  transmitted  with 
reference  to  the  planes  of  crystallization. 

It  is  found,  moreover,  that  the  slightest  difference  in  ho- 
mogeneity in  the  same  metal  enables  it  when  heated  to  pro- 
duce a  thermo-electric  current,  and  that  metals  in  a  state  of 
fusion,  in  which  state  they  may  be  presumed  to  be  homoge- 
neous throughout,  give  no  thermo-electric  current:  thus,  hot 
in  contact  with  cold  mercury  has  been  shown  by  Matteucci 
to  give  no  thermo-electric  current,  and  the  same  is  the  case 
with  portions  of  fused  bismuth  unequally  heated. 

The  fact  that  the  molecular  structure  or  arrangement  of  a 
body  influences — indeed  I  may  say  determines — its  conduct- 
ing power,  is  by  no  means  explained  by  the  theory  of  a  fluid ; 
but  if  electricity  be  only  a  transmission  of  force  or  motion, 
the  influence  of  the  molecular  state  is  just  what  would  be 
expected.  Carbon,  in  a  transparent  crystalline  state,  as  dia- 
mond, is  as  perfect  a  non-conductor  as  we  know ;  while  in  an 
opaque  amorphous  state,  as  graphite  or  charcoal,  it  is  one  of 
the  best  conductors  :  thus,  in  the  one  state,  it  transmits  light 
and  stops  electricity,  in  the  other  it  transmits  electricity  and 
stops  light. 

It  is  a  circumstance  worthy  of  remark,  that  the  arrange- 
ment of  molecules,  which  renders  a  solid  body  capable  of 
5 


98  CORRELATION   OF  PHYSICAL   FORCES. 

transmitting  light,  is  most  unfavourable  to  its  transmission  of 
electricity,  transparent  solids  being  very  imperfect  conductors 
of  electricity ;  so  all  gases  readily  transmit  light,  but  are 
amongst  the  worst  conductors  of  electricity,  if,  indeed,  prop- 
erly speaking,  they  can  be  said  to  conduct  at  all. 

The  conduction  of  electricity  by  different  classes  of  bodies 
has  been  generally  regarded  as  a  question  of  degree :  thus 
metals  were  viewed  as  perfect  conductors,  charcoal  less  so, 
water  and  other  liquids  as  imperfect  conductors,  &c.  But, 
in  fact,  though  between  one  metal  and  another  the  mode  of 
transmission  may  be  the  same  and  the  difference  one  of  de- 
gree, a  different  molecular  effect  obtains,  when  we  contrast 
metals  with  electrolytic  liquids  and  these  with  gases. 

Attenuated  gases  may  be,  in  one  sense,  regarded  as  non- 
conductors, in  another,  as  conductors ;  thus  if  gold-leaves  be 
made  to  diverge,  by  electrical  repulsion,  in  air  at  ordinary 
pressure,  they  in  a  short  time  collapse  ;  while  in  highly-rare- 
fied air,  or  what  is  commonly  termed  a  vacuum,  they  remain 
divergent  for  days  ;  and  yet  electricity  of  a  certain  degree  of 
tension  passes  readily  across  attenuated  air,  and  with  diffi- 
culty across  air  of  ordinary  density. 

Again,  where  the  electrical  terminals  are  brought  to  a 
state  of  visible  ignition,  there  are  symptoms  of  the  transmis- 
sion of  electricity  of  low  tension  across  gases ;  but  no  such 
effects  have  been  detected  at  lower  temperatures.  All  this 
presents  a  strong  argument  in  favour  of  the  transmission  of 
electricity  across  gases  being  effected  by  the  disruptive  dis- 
charge, and  not  by  a  conduction  similar  to  that  which  takes 
place  with  metals  or  with  electrolytes. 

The  ordinary  attractions  and  repulsions  of  electrified 
bodies  present  no  more  difficulty  when  regarded  as  being  pro- 
duced by  a  change  in  the  state  or  relations  of  the  matter  af- 
fected, than  do  the  attractions  of  the  earth  by  the  sun,  or  of 
a  leaden  ball  by  the  earth ;  the  hypothesis  of  a  fluid  is  not 
considered  necessary  for  the  latter,  and  need  not  be  so  for  the 


ELECTRICITY.  99 

former  class  of  phenomena.  How  the  phenomena  are  pro- 
duced to  which  the  term  attraction  is  applied  is  still  a  mys- 
tery. Newton,  speaking  of  it,  says,  '  What  I  call  attraction 
may  be  performed  by  impulse,  or  by  some  other  means  un- 
known to  me.  I  use  that  word  here  to  signify  only  in  gen- 
eral any  force  by  which  bodies  tend  towards  one  another, 
whatsoever  be  the  cause.'  If  we  suppose  a  fluid  to  act  in  at- 
tractions and  repulsions,  the  imponderable  fluid  must  drag  or 
push  the  matter  with  it:  thus  when  we  feel  a  stream  of 
air  rushing  from  an  electrified  metallic  point,  each  molecule 
of  air  contiguous  to  the  point  being  repelled,  another  takes 
its  place,  which  is  in  its  turn  repelled : — how  does  a  hypo- 
thetic fluid  assist  us  here  ?  If  we  say  the  electrical  fluid  re- 
pels itself,  or  the  same  electricity  repels  itself,  we  must  go 
farther  and  assert,  that  it  not  only  repels  itself,  but  either 
communicates  its  repulsive  force  to  the  particles  of  the  air,  or 
carries  with  it  the  particle  of  air  in  its  passage.  Is  it  not 
more  easy  to  assume  that  the  particle  of  air  is  in  such  a  state 
that  the  ordinary  forces  which  keep  it  in  equilibrium  are  dis- 
turbed by  the  electrical  force,  or  force  in  a  definite  direction 
communicated  to  it,  and  that  thus  each  particle  in  turn  re- 
cedes from  the  point?  As  this  latter  force  is  increased,  not 
only  does  the  particle  of  air  which  was  contiguous  to  the  me- 
tallic point  recede,  but  the  cohesion  of  the  extreme  particles 
of  metal  may  be  overcome  to  such  an  extent  that  these  are 
detached,  and  the  brush  or  spark  may  consist  wholly  or  in 
part  of  minute  particles  of  the  metal  itself  thrown  off.  Of 
this  there  is  some  evidence,  though  the  point  can  hardly  be 
considered  as  proved.  A  similar  effect  undoubtedly  takes 
place  with  voltaic  electricity,  acting  upon  a  terminal  im- 
mersed in  a  liquid ;  thus  if  metallic  terminals  of  a  powerful 
voltaic  battery  be  immersed  in  water,  metal,  or  the  oxide  of 
metal,  is  forcibly  detached,  producing  great  heat  at  the  point 
of  disruption. 

If  we  apply  ourselves  to  the  effect  of  electricity  in  the 


100  COEEELATION   OF   PHYSICAL   FOECES. 

animal  economy,  we  find  that  the  first  rationale  given  of  the 
convulsive  effect  produced  by  transmission  through  the  living 
or  recently  killed  animal  was,  that  electricity  itself,  something 
substantive,  passed  rapidly  through  the  body,  and  gave  rise 
to  the  contractions  ;  step  by  step  we  are  now  arriving  at  the 
conviction  that  consecutive  particles  of  the  nerves  and  mus- 
cles are  affected.  Thus  the  contractions  which  the  prepared 
leg  of  a  frog  undergoes  at  the  moment  it  is  submitted  to  a 
voltaic  current,  cease  after  a  time  if  the  current  be  contin- 
ued, and  are  renewed  on  breaking  the  circuit,  i.  e.  at  the  mo- 
ment when  the  current  ceases  to  traverse  it.  The  excitabil- 
ity of  a  nerve,  moreover,  or  its  power  of  producing  muscular 
contraction,  is  weakened  or  destroyed  by  the  transmission  of 
electricity  in  one  direction,  while  the  excitability  is  increased 
by  the  transmission  of  electricity  in  the  opposite  direction ; 
showing  that  the  fibre  or  matter  itself  of  the  nerve  is  changed 
by  electrisation,  and  changed  in  a  manner  bearing  a  direct 
relation  to  the  other  effects  produced  by  electricity. 

Portions  of  muscle  and  of  nerve  present  different  electri- 
cal states  with  reference  to  other  portions  of  the  same  muscle 
or  nerve  ;  thus  the  external  part  of  a  muscle  bears  the  same 
relation  to  the  internal  part  as  platinum  does  to  zinc  in  the 
voltaic  battery  ;  and  delicate  galvanoscopes  will  show  electri- 
cal effects  when  interposed  in  a  conducting  circuit  connecting 
the  surface  of  a  nerve  with  its  interior  portions.  Matteucci 
has  proved  that  a  species  of  voltaic  pile  may  be  formed  by  a 
series  of  slices  of  muscle,  so  arranged  that  the  external  part 
of  one  slice  may  touch  the  internal  part  of  the  next,  and 
so  on. 

Lastly,  the  magnetic  effects  produced  by  electricity  also 
show  a  change  in  the  molecular  state  of  the  magnetic  sub- 
stance affected ;  as  we  shall  see  when  the  subject  of  magnet- 
ism is  discussed. 

I  have  taken  in  succession  all  the  known  classes  of  elec- 
trical phenomena ;  and,  as  far  as  I  am  aware,  there  is  not  an 


ELECTRICITY.  101 

electrical  effect,  where,  if  a  close  investigation  be  instituted, 
and  the  materials  chosen  in  a  state  for  exhibiting  minute 
changes,  evidence  of  molecular  change  will  not  be  detected  ; 
thus,  excepting  those  cases  where  infinite simally  small  quan- 
tities of  matter  are  acted  on,  and  our  means  of  detection  fail, 
electrical  effects  are  known  to  us  only  as  changes  of  ordinary 
matter.  It  seems  to  me  as  easy  to  imagine  these  changes  to 
be  effected  by  a  force  acting  in  definite  directions,  as  by  a 
fluid  which  has  no  independent  or  sensible  existence,  and 
which,  it  must  be  assumed,  is  associated  with,  or  exerts  a 
force  acting  upon  ordinary  matter,  or  matter  of  a  different 
order  from  the  supposed  fluid.  As  the  idea  of  the  hypothetic 
fluid  is  pursued,  it  gradually  vanishes,  and  resolves  itself  into 
the  idea  of  force.  The  hypothesis  of  matter  without  weight 
presents  in  itself,  as  I  believe,  fatal  objections  to  the  theories 
of  electrical  fluids,  which  are  entirely  removed  by  viewing 
electricity  as  force,  and  not  as  matter. 

If  it  be  said  that  the  effects  we  have  been  considering 
may  still  be  produced  by  a  fluid,  and  that  this  fluid  acts  upon 
ordinary  matter  in  certain  cases,  polarising  the  matter  af- 
fected or  arranging  its  particles  in  a  definite  direction,  whilst 
in  others,  by  its  attractive  or  repulsive  force,  it  carries  with 
it  portions  of  matter ;  yet,  if  the  fluid  in  itself  be  incapable 
of  recognition  by  any  test,  if  it  be  only  evidenced  by  the 
changes  which  it  operates  in  ponderable  matter,  the  words 
fluid  and  force  become  identical  in  meaning ;  we  may  as  well 
say  that  the  attraction  of  gravitation  or  weight  is  occasioned 
by  a  fluid,  as  that  electrical  changes  are  so. 

When,  as  is  constantly  done  in  common  parlance,  a  house 
is  said  to  be  struck,  windows  broken,  metals  fused  or  dissipa- 
ted by  the  electrical  fluid,  are  not  the  expressions  used  such 
as,  if  not  sanctioned  by  habit,  would  seem  absurd  ?  In  all 
the  cases  of  injury  done  by  lightning  there  is  no  fluid  per- 
ceptible ;  the  so-called  sulphurous  odour  is  either  ozone  de- 
veloped by  the  action  of  electricity  on  atmospheric  air,  or  the 


102  COKEELATTON   OF   PHYSICAL   FOKCES. 

vapour  of  some  substance  dissipated  by  the  discharge  ;  on  the 
other  hand,  it  seems  more  consonant  with  experience  to  re- 
gard these  effects  as  produced  by  force,  as  we  have  analogous 
effects  produced  by  admitted  forces,  in  cases  where  no  one 
would  invoke  the  aid  of  a  hypothetic  fluid  for  explanation. 
For  instance,  glasses  may  be  broken  by  electrical  discharges  ; 
so  may  they  by  sonorous  vibrations.  Metals  electrified  or 
magnetised  will  emit  a  sound ;  so  they  will  if  struck,  or  if  a 
musical  note  with  which  they  can  vibrate  in  unison  be  sounded 
near  to  them. 

Even  chemical  decomposition,  in  cases  of  feeble  affinity, 
may  be  produced  by  purely  mechanical  effects.  A  number  of 
instances  of  this  have  been  collected  by  M.  Becquerel ;  and 
substances  whose  constituents  are  held  together  by  feeble  af- 
finities, such  as  iodide  of  nitrogen  and  similar  compounds,  are 
decomposed  by  the  vibration  occasioned  by  sound. 

If,  instead  of  being  regarded  as  a  fluid  or  imponderable 
matter  sui  generis,  electricity  be  regarded  as  the  motion  of  an 
ether,  equal  difficulties  are  encountered.  Assuming  ether  to 
pervade  the  pores  of  all  bodies,  is  the  ether  a  conductor  or 
non-conductor  ?  If  the  latter — that  is,  if  the  ether  be  incapa- 
ble of  transmitting  the  electrical  wave — the  ethereal  hypothe- 
sis of  electricity  necessarily  falls ;  but  if  the  motion  of  the 
ether  constitute  what  we  call  conduction  of  electricity,  then 
the  more  porous  bodies,  or  those  most  permeable  by  the 
ether,  should  be  the  best  conductors.  But  this  is  not  the  case. 
If,  again,  the  metal  and  the  air  surrounding  it  are  both  per- 
vaded by  ether,  why  should  the  electrical  wave  affect  the 
ether  in  the  metal,  and  not  stir  that  in  the  gas  ?  To  support 
an  ethereal  hypothesis  of  electricity,  many  additional  and 
hardly  reconcilable  hypotheses  must  be  imported. 

The  fracture  and  comminution  of  a  non-conducting  body, 
the  fusion  or  dispersion  of  a  metallic  wire  by  the  electrical 
discharge,  are  effects  equally  difficult  to  conceive  upon  the 
hypothesis  of  an  ethereal  vibration,  as  upon  that  of  a  fluid, 


ELECTRICITY.          *  103 

but  are  necessary  results  of  the  sudden  subversion  of  mole- 
cular polarisation,  or  of  a  sudden  or  irregular  vibratory  move- 
ment of  the  matter  itself.  We  see  similar  effects  produced 
by  sonorous  vibrations,  which  might  be  called  conduction 
and  non-conduction  of  sound.  One  body  transmits  sound  ea- 
sily, another  stops  or  deadens  it,  as  it  is  termed — i.  e.  dis- 
perses the  vibrations,  instead  of  continuing  them  in  the  same 
direction  as  the  primary  impulse  ;  and  solid  bodies  may,  as 
has  been  above  observed,  be  shivered  by  sudden  impulses  of 
sound  in  those  cases  where  all  the  parts  of  the  body  cannot 
uniformly  carry  on  the  undulatory  motion. 

The  progressive  stages  in  the  History  of  Physical  Philoso- 
phy will  account  in  a  great  measure  for  the  adoption  by  the 
early  electricians  of  the  theories  of  fluids. 

The  ancients,  when  they  witnessed  a  natural  phenomenon, 
removed  from  ordinary  analogies,  and  unexplained  by  any 
mechanical  action  known  to  them,  referred  it  to  a  soul,  a 
spiritual  or  preternatural  power :  thus  amber  and  the  magnet 
were  supposed  by  Thales  to  have  a  soul ;  the  functions  of 
digestion,  assimilation,  &c.,  were  supposed  by  Paracelsus  to 
be  effected  by  a  spirit  (the  Archaeus) .  Air  and  gases  were 
also  at  first  deemed  spiritual,  but  subsequently  became  invest- 
ed with  a  more  material  character ;  and  the  word  gas,  from 
</eist}  a  ghost  or  spirit,  affords  us  an  instance  of  the  gradual 
transmission  of  a  spiritual  into  a  physical  conception. 

The  establishment  by  Torricelli  of  the  ponderable  charac- 
ter of  air  and  gas,  showed  that  substances  which  had  been 
deemed  spiritual  and  essentially  different  from  ponderable 
matter  were  possessed  of  its  attributes.  A  less  superstitious 
mode  of  reasoning  ensued,  and  now  aeriform  fluids  were 
shown  to  be  analogous  in  many  of  their  actions  to  liquids  or 
known  fluids.  A  belief  in  the  existence  of  other  fluids,  differ- 
ing from  air  as  this  differed  from  water,  grew  up,  and  when 
a  new  phenomenon  presented  itself,  recourse  was  had  to  a 
hypothetic  fluid  for  explaining  the  phenomenon  and 


104  COEEELATION   OF   PHYSICAL   FORCES. 

ing  it  with  others  ;  the  mind  once  possessed  of  the  idea  of  a 
fluid,  soon  invested  it  with  the  necessary  powers  and  proper- 
ties, and  grafted  upon  it  a  luxurious  vegetation  of  imaginary 
offshoots. 

In  what  I  am  here  throwing  out,  I  wish  to  guard  myself 
from  being  supposed  to  state  that  the  theory,  historically 
viewed,  followed  exactly  the  dates  of  the  discoveries  which 
were  effectual  in  changing  its  character ;  sometimes  a  dis- 
covery precedes,  at  other  times  it  succeeds  to  a  change  in  the 
general  course  of  thought ;  sometimes,  and  perhaps  most 
frequently,  it  does  both — i.  e.  the  discovery  is  the  result  of  a 
tendency  of  the  age  and  of  the  continually  improved  methods 
of  observation,  and  when  made,  it  strengthens  and  extends  the 
views  which  have  led  to  it.  I  think  the  phases  of  thought 
which  physical  philosophers  have  gone  through,  will  be  found 
generally  such  as  I  have  indicated,  and  that  the  gradual  ac- 
cumulation of  discoveries  which  has  taken  place  during  the 
more  recent  periods,  by  showing  what  effects  can  be  produced 
by  dynamical  causes  alone,  is  rapidly  tending  to  a  general 
dynamical  theory  into  which  that  of  the  imponderable  fluids 
promises  ultimately  to  merge. 

Commencing  with  electricity  as  an  initiating  force,  we 
get  motion  directly  produced  by  it  in  various  forms  ;  for  in- 
stance, in  the  attraction  and  repulsion  of  bodies,  evidenced  by 
mobile  electrometers,  such  as  that  of  Cuthbertson,  where 
large  masses  are  acted  on ;  the  rotation  of  the  fly-wheel, 
another  form  of  electrical  repulsion,  and  the  deflection  of 
the  galvanometer  needle,  are  also  modes  of  palpable,  visible 
motion. 

It  would  follow,  from  the  reasoning  in  this  essay,  that 
when  electricity  performs  any  mechanical  work  which  does 
not  return  to  the  machine,  electrical  power  is  lost.  It  would 
be  unsuitable  to  the  scope  of  this  work  to  give  the  mathemati- 
cal labours  of  M.  Clausius  and  others  here  ;  but  the  follow- 
ing experiment,  which  I  devised  for  making  the  result  evi- 


ELECTRICITY.  105 

dent  to  an  audience  at  the  Royal  Institution,  will  form  a 
useful  illustration  : — A  Leyden  jar,  of  one  square  foot  coated 
surface,  has  its  interior  connected  with  a  Cuthbertson's  elec- 
trometer, between  which  and  the  outer  coating  of  the  jar 
are  a  pair  of  discharging  balls  fixed  at  a  certain  distance 
(about  half  an  inch  apart).  Between  the  Leyden  jar  and 
the  prime  conductor  is  inserted  a  small  unit  jar  of  nine  inches 
surface,  the  knobs  of  which  are  0*2  inch  apart. 

The  balance  of  the  electrometer  is  now  fixed  by  a  stiff 
wire  inserted  between  the  attracting  knobs,  and  the  Leyden 
jar  charged  by  discharges  from  the  unit  jar.  After  a  certain 
number  of  these,  say  twenty,  the  discharge  of  the  large  jar 
takes  place  across  the  half-inch  interval.  This  may  be 
viewed  as  the  expression  of  electrical  power  received  from 
the  unit  jar.  The  experiment  is  now  repeated,  the  wire 
between  the  balls  having  been  removed,  and  therefore  the 
'  tip/  or  the  raising  of  the  weight,  is  performed  by  the  electri- 
cal repulsion  and  attraction  of  the  two  pairs  of  balls.  At 
twenty  discharges  of  the  unit  jar  the  balance  is  subverted, 
and  one  attracting  knob  drops  upon  the  other ;  but  no  dis- 
charge takes  place,  showing  that  some  electricity  has  been  lost 
or  converted  into  the  mechanical  power  which  raised  the 
balance. 

By  another  mode  of  expression,  the  electricity  may  be 
supposed  to  be  masked  or  analogous  to  latent  heat,  and  it 
would  be  restored  if  the  ball  were  brought  back  without  dis- 
charge by  extraneous  force.  If  the  discharge  or  other  elec- 
trical effects  were  the  same  in  both  cases,  then,  since  the 
raising  of  the  ball  or  weight  is  an  extra  mechanical  effort, 
and  since  the  weight  is  capable  by  its  fall  of  producing  elec- 
tricity, heat,  or  other  force,  it  would  seem  that  force  could  be 
got  out  of  nothing,  or  perpetual  motion  obtained. 

The  above  experiment  is  suggestive  of  others  of  a  similar 
character,  which  may  be  indefinitely  varied.  Thus  I  have 
found  that  two  balls  made  to  diverge  by  electricity  do  not 
5* 


106  CORRELATION    OF   PHYSICAL   FORCES. 

give  to  an  electrometer  the  same  amount  of  electricity  as  they 
do  if,  whilst  similarly  electrified,  they  are  kept  forcibly  to- 
gether. This  experiment  is  the  converse  of  the  former  one. 
There  is  an  advantage  in  electrical  experiments  of  this  class 
as  compared  with  those  on  heat,  viz.  that  though  there  is  no 
perfect  insulation  for  electricity,  yet  our  means  of  insula- 
tion are  immeasurably  superior  to  any  attainable  for  heat. 

Electricity  directly  produces  lieat,  as  shown  in  the  ignited 
wire,  the  electric  spark,  and  the  voltaic  arc :  in  the  latter 
the  most  intense  heat  with  which  we  are  acquainted — so  in- 
tense, indeed,  that  it  cannot  be  measured,  as  every  sort  of 
matter  is  dissipated  by  it. 

In  the  phenomenon  of  electrical  ignition,  as  shown  by  a 
heated  conjunctive  wire,  the  relation  of  force  and  resistance, 
and  the  correlative  character  of  the  two  forces,  electricity  and 
heat,  are  strikingly  demonstrated.  Let  a  thin  wire  of  plati- 
num join  the  terminals  of  a  voltaic  battery  of  suitable  power, 
the  wire  will  be  ignited,  and  a  certain  amount  of  chemical 
action  will  take  place  in  the  cells  of  the  battery — a  definite 
quantity  of  zinc  being  dissolved  and  of  hydrogen  eliminated 
in  a  given  time.  If  now  the  platinum  wire  be  immersed  in 
water,  the  heat  will,  from  the  circulating  currents  of  the 
liquid,  be  more  rapidly  dissipated,  and  we  shall  instantly  find 
that  the  chemical  action  in  the  battery  will  be  increased,  more 
zinc  will  be  dissolved,  and  more  hydrogen  eliminated  for  the 
same  time  ;  the  heat  being  conveyed  away  by  the  water, 
more  chemical  action  is  required  to  generate  it,  just  as  more 
fuel  is  required  in  proportion  as  evaporation  is  more 
rapid. 

Reverse  the  experiment,  and  instead  of  placing  the  wire 
in  water,  place  it  in  the  flame  of  a  spirit  lamp,  so  that  the 
force  of  heat  meets  with  greater  resistance  to  its  dissipation. 
We  now  find  that  the  chemical  action  is  less  than  in  the  first 
or  normal  experiment.  If  the  wire  be  placed  in  other  differ- 
ent gaseous  or  liquid  media,  we  shall  find  that  the  chemical 


ELECTRICITY.  107 

action  of  the  battery  will  be  proportioned  to  the  facility  with 
which  the  heat  is  circulated  or  radiated  by  these  media,  and 
we  thus  establish  an  alternating  reciprocity  of  action  between 
these  two  forces :  a  similar  reciprocity  may  be  established 
between  electricity  and  motion,  magnetism  and  motion,  and 
so  of  other  forces.  If  it  cannot  be  realised  with  all,  it  is 
probably  because  we  have  not  yet  eliminated  interfering  ac- 
tions. If  we  carefully  think  over  the  matter,  we  shall,  unless 
I  am  much  mistaken,  arrive  at  the  conclusion  that  it  cannot 
be  otherwise,  unless  it  be  supposed  that  a  force  can  arise  from 
nothing — can  exist  without  antecedent  force. 

In  the  phenomenon  of  the  voltaic  arc,  the  electric  spark, 
&c.,  to  which  I  have  already  adverted,  electricity  directly 
produces  light  of  the  greatest  known  intensity.  It  directly 
produces  magnetism,  as  shown  by  Oersted,  who  first  distinctly 
proved  the  connection  between  electricity  and  magnetism. 
These  two  forces  act  upon  each  other,  not  in  straight  lines, 
as  all  other  known  forces  do,  but  in  a  rectangular  direction ; 
that  is,  bodies  affected  by  dynamic  electricity,  or  the  conduits 
of  an  electric  current,  tend  to  place  magnets  at  right  angles 
to  them  ;  and,  conversely,  magnets  tend  to  place  bodies  con- 
ducting electricity  at  right  angles  to  them.  Thus  an  electric 
current  appears  to  have  a  magnetic  action,  in  a  direction 
cutting  its  own  at  right  angles ;  or,  supposing  its  section  to 
be  a  circle,  tangential  to  it :  if,  then,  we  reverse  the  position, 
and  make  the  electric  current  form  a  series  of  tangents  to  an 
imaginary  cylinder,  this  cylinder  should  be  a  magnet.  This 
is  effected  in  practice  by  coiling  a  wire  as  a  helix  or  spiral, 
and  this,  when  conducting  an  electrical  current,  is  to  all  in- 
tents and  purposes  a  magnet.  A  soft  iron  core  placed  within 
such  a  helix  has  the  property  of  concentrating  its  power,  and 
then  we  can,  by  connection  or  disconnection  with  the  source 
of  electricity,  instantly  make  or  unmake  a  most  powerful 
magnet. 

We  may  figure  to  the  mind  electrified  and  magnetised 


108  CORRELATION   OF  PHYSICAL  FORCES. 

matter,  as  lines  of  which  the  extremities  repel  each  other  in 
a  definite  direction;  thus,  if  a  line  A  B  represent  a  wire 
affected  by  electricity,  and  superposed  on  c  D  a  wire  affected 
by  magnetism,  the  extreme  points  A  and  B  will  be  repelled  to 
the  farthest  distances  from  the  points  c  and  D,  and  the  line  A 
B  be  at  right  angles  to  the  line  c  D  ;  and  so,  if  the  lines  be 
subdivided  to  any  extent,  each  will  have  two  extremities  or 
poles  repulsive  of  those  of  the  other.  If  the  line  of  matter 
affected  by  electricity  be  a  liquid,  and  consequently  have 
"entire  mobility  of  particles,  a  continuous  movement  will  be 
produced  by  magnetism,  each  particle  successively  tending, 
as  it  were,  to  fly  off  at  a  tangent  from  the  magnet :  thus, 
place  a  flat  dish  containing  acidulated  water  on  the  poles  of  a 
powerful  magnet,  immerse  the  terminals  of  a  voltaic  battery 
in  the  liquid  just  above  the  magnetic  poles,  so  that  the  lines 
of  electricity  and  of  magnetism  coincide  ;  the  water  will  now 
assume  a  movement  at  right  angles  to  this  line,  flowing  con- 
tinously,  as  if  blown  by  an  equatorial  wind,  which  may  be 
made  east  or  west  with  reference  to  the  magnetic  poles  by 
altering  the  direction  of  the  electrical  current :  a  similar  effect 
may  be  produced  with  mercury.  These  cases  afford  an 
additional  argument  to  those  previously  mentioned  of  the 
particles  of  matter  being  affected  by  the  forces  of  electricity 
and  magnetism  in  a  way  irreconcilable  with  the  fluid  or 
ethereal  hypothesis. 

The  representation  of  transverse  direction  by  magnetism 
and  electricity  appears  to  have  led  Coleridge  to  parallel  it  by 
the  transverse  expansion  of  matter,  or  length  and  breadth, 
though  he  injured  the  parallel  by  adding  galvanism  as  depth : 
whether  a  third  force  exists  which  may  bear  this  relation  to 
electricity  and  magnetism  is  a  question  upon  which  we  have 
no  evidence. 

The  ratio  which  the  attractive  magnetic  force  produced 
bears  to  the  electric  current  producing  it  has  been  investigat- 
ed by  many  experimentalists  and  mathematicians.  The  data 


ELECTRICITY.  109 

are  so  numerous  and  so  variable,  that  it  is  difficult  to  arrive 
at  definite  results.  Thus  the  relative  size  of  the  coil  and  the 
iron,  the  temper  or  degree  of  hardness  of  the  latter,  its  shape, 
or  the  proportions  of  length  to  diameter,  the  number  of  coils 
surrounding  it,  the  conducting  power  of  the  metal  of  which 
the  coils  are  formed,  the  size  of  the  keeper  or  iron  in  which 
magnetism  is  induced,  the  degree  of  constancy  of  the  bat- 
tery, &c.,  complicate  the  experiments. 

The  most  trustworthy  general  relation  which  has  been  as- 
certained is,  that  the  magnetic  attraction  is  as  the  square  of 
the  electric  force ;  a  result  due  to  the  researches  of  Lenz  and 
Jacobi,  and  also  of  Sir  "W".  S.  Harris. 

Lastly,  electricity  produces  chemical  affinity ;  and  by  its 
agency  we  are  enabled  to  obtain  effects  of  analysis  or  synthe- 
sis with  which  ordinary  chemistry  does  not  furnish  us.  Of 
these  effects  we  have  examples  in  the  brilliant  discoveries,  by 
Davy,  of  the  alkaline  metals,  and  in  the  peculiar  crystalline 
compounds  made  known  by  Crosse  and  Becquerel. 


V.—  LIGHT. 

IN  entering  on  the  subject  of  LIGHT,  it  will  be  well  to  de- 
scribe briefly,  and  in  a  manner  as  far  as  may  be  inde- 
pendent of  theory,  the  effects  to  which  the  term  polarisation 
has  been  applied. 

When  light  is  reflected  from  the  surface  of  water,  glass, 
or  many  other  media,  it  undergoes  a  change  which  disables  it 
from  being  again  similarly  reflected  in  a  direction  at  right 
angles  to  that  at  which  it  has  been  originally  reflected. 
Light  so  affected  is  said  to  be  polarised ;  it  will  always  be 
capable  of  being  reflected  in  planes  parallel  to  the  plane  in 
which  it  has  been  first  reflected,  but  incapable  of  being  re- 
flected in  planes  at  right  angles  to  that  plane.  At  planes 
having  a  direction  intermediate  between  the  original  plane  of 
reflection,  and  a  plane  at  right  angles  to  it,  the  light  will  be 
capable  of  being  partially  reflected,  and  more  or  less  so  ac- 
cording as  the  direction  of  the  second  plane  of  reflection  is 
more  or  less  coincident  with  the  original  plane.  Light,  again, 
when  passed  through  a  crystal  of  Iceland  spar,  is  what  is 
termed  doubly  refracted,  i.  e.  split  into  two  divisions  or  beams, 
each  having  half  the  luminosity  of  the  original  incident  light ; 
each  of  these  beams  is  polarised  in  planes  at  right  angles  to 
each  other  ;  and  if  they  be  intercepted  by  the  mineral  tour- 
maline, one  of  them  is  absorbed,  so  that  only  one  polarised 
beam  emerges.  Similar  effects  may  be  produced  by  certain 


LIGHT.  Ill 

other  reflections  or  refractions.  A  ray  of  light  once  polaris- 
ed in  a  certain  plane  continues  so  affected  throughout  its 
whole  subsequent  course  ;  and  at  any  indefinite  distance  from 
the  point  where  it  originally  underwent  the  change,  the  di- 
rection of  the  plane  will  be  the  same,  provided  the  media 
through  which  it  is  transmitted  be  air,  water-,  or  certain  other 
transparent  substances  which  need  not  be  enumerated.  If, 
however,  the  polarised  ray,  instead  of  passing  through  water, 
be  made  to  pass  through  oil  of  turpentine,  the  definite  direc- 
tion in  which  it  is  polarised  will  be  found  to  be  changed ;  and 
the  change  of  direction  will  be  greater  according  to  the 
length  of  the  column  of  interposed  liquid.  Instead  of  being 
an  uniform  plane,  it  will  have  a  curvilinear  direction, 
similar  to  that  which  a  strip  of  card  would  have  if  forced 
along  two  opposite  grooves  of  a  rifle-barrel.  This  curious 
effect  is  produced  in  different  degrees  by  different  media. 
The  direction  also  varies  ;  the  rotation,  as  it  is  termed,  being 
sometimes  to  the  right  hand  and  sometimes  to  the  left,  accord- 
ing to  the  peculiar  molecular  character  of  the  medium  through 
which  the  polarised  ray  is  transmitted. 

Light  is,  perhaps,  that  mode  of  force  the  reciprocal  rela- 
tions of  which  with  the  others  have  been  the  least  traced 
out.  Until  the  discoveries  of  Niepce,  Daguerre,  and  Talbot, 
very  little  could  be  definitely  predicated  of  the  action  of  light 
in  producing  other  modes  of  force.  Certain  chemical  com- 
pounds, among  which  stand  pre-eminent  the  salts  of  silver, 
have  the  property  of  suffering  decomposition  when  exposed 
to  light.  If,  for  instance,  recently  formed  chloride  of  silver 
be  submitted  to  luminous  rays,  a  partial  decomposition  en- 
sues ;  the  chlorine  is  separated  and  expelled  by  the  action  of 
light,  and  the  silver  is  precipitated.  By  this  decomposition 
the  colour  of  the  substance  changes  from  white  to  blue.  If 
now,  paper  be  impregnated  with  chloride  of  silver,  which  can 
be  done  by  a  simple  chemical  process,  then  partially  covered 
with  an  opaque  substance,  a  leaf  for  example,  and  exposed  to 


112  CORRELATION   OF  PHYSICAL   FORCES. 

a  strong  light,  the  chloride  will  be  decomposed  in  all  those 
parts  of  the  paper  where  the  light  is  not  intercepted,  and  we 
shall  have,  by  the  action  of  light,  a  white  image  of  the  leaf 
on  a  purple  ground.  If  similar  paper  be  placed  in  the  focus 
of  a  lens  in  a  camera-obscura,  the  objects  there  depicted  will 
decompose  the  chloride,  just  in  the  proportion  in  which  they 
are  luminous  ;  and  thus,  as  the  most  luminous  parts  of  the  im- 
age will  most  darken  the  chloride,  we  shall  have  a  picture  of 
the  objects  with  reversed  lights  and  shadows.  The  picture 
thus  produced  would  not  be  permanent,  as  subsequent  expos- 
ure would  darken  the  light  portion  of  the  picture  :  to  fix  it, 
the  paper  must  be  immersed  in  a  solution  which  has  the  pro- 
perty of  dissolving  chloride  of  silver,  but  not  metallic  silver. 
Iodide  of  potassium  will  effect  this ;  and  the  paper  being 
washed  and  dried  will  then  preserve  a  permanent  image  of 
the  depicted  objects.  This  was  the  first  and  simple  process 
of  Mr.  Talbot ;  but  it  is  defective  as  to  the  purposes  aimed 
at,  in  many  points.  First,  it  is  not  sufficiently  sensitive,  re- 
quiring a  strong  light  and  a  long  time  to  produce  an  image  ; 
secondly,  the  lights  and  shadows  are  reversed ;  and  thirdly, 
the  coarse  structure  of  the  finest  paper  does  not  admit  of  the 
delicate  traces  of  objects  being  distinctly  impressed.  These 
defects  have  been  to  a  great  extent  remedied  by  a  process 
subsequently  discovered  by  Mr.  Talbot,  and  which  bears  his 
name,  and  which  has  led  to  the  collodion  process,  and  others 
unnecessary  to  be  detailed  here. 

The  photographs  of  M.  Daguerre,  with  which  all  are  now 
familiar,  are  produced  by  holding  a  plate  of  highly-polished 
silver  over  iodine.  A  thin  film  of  iodide  of  silver  is  thus 
formed  on  the  surface  of  the  metal ;  and  when  these  iodized 
plates  are  exposed  in  the  camera,  a  chemical  .alteration  takes 
place.  The  portions  of  the  plate  on  which  the  light  has  im- 
pinged part  with  some  of  the  iodine,  or  are  otherwise  changed 
— for  the  theory  is  somewhat  doubtful — soas  to  be  capable 
of  ready  amalgamation.  When,  therefore,  the  plate  is  placed 


LIGHT.  113 

over  the  vapour  of  heated  mercury,  the  mercury  attaches  it- 
self to  the  portions  affected  by  light,  and  gives  them  a  white 
frosted  appearance ;  the  intermediate  tints  are  less  affected, 
and  those  parts  where  no  light  has  fallen,  by  retaining 
their  original  polish,  appear  dark;  the  iodide  of  silver  is 
then  washed  off  by  hyposulphite  of  soda,  which  has  the 
property  of  dissolving  it,  and  there  remains  a  picture 
in  which  the  lights  and  shadows  are  as  in  nature,  and  the 
molecular  uniformity  of  the  metallic  surface  enables  the 
most  microscopic  details  to  be  depicted  with  perfect  accu- 
racy. By  using  chloride  of  iodine,  or  bromide  of  iodine, 
instead  of  iodine,  the  equilibrium  of  chemical  forces  is  ren- 
dered still  more  unstable,  so  that  images  may  be  taken  in  an 
indefinitely  short  period — a  period  practically  instantaneous. 

It  would  be  foreign  to  the-  object  of  this  essay  to  enter 
upon  the  many  beautiful  details  into  which  the  science  of 
photography  has  branched  out,  and  the  many  valuable  discov- 
eries and  practical  applications  to  which  it  has  led.  The 
short  statement  I  have  given  above  is  perhaps  superfluous,  as, 
though  they  were  new  and  surprising  at  the  period  when  these 
Lectures  were  delivered,  photographic  processes  have  now  be- 
come familiar,  not  only  to  the  cultivator  of  science,  but  to 
the  artist  and  amateur  ;  the  important  point  for  consideration 
here  is  that  light  will  chemically  or  molecularly  affect  mat- 
ter. Not  only  will  the  particular  compounds  above  selected 
as  instances  be  changed  by  the  action  of  light ;  but  a  vast 
number  of  substances,  both  elementary  and  compound,  are 
notably  affected  by  this  agent,  even  those  apparently  the  most 
unalterable  in  character,  such  as  metals :  so  numerous,  in- 
deed, are  the  substances  affected,  that  it  has  been  supposed, 
not  without  reason,  that  matter  of  every  description  is  altered 
by  exposure  to  light. 

The  permanent  impression  stamped  on  the  molecules  of 
matter  by  light  can  be  made  to  repeat  itself  by  the  same 
agency,  but  always  with  decreasing  force.  Thus  a  photo- 


114  CORRELATION   OF   PHYSICAL   FORCES. 

graph  placed  opposite  a  camera  containing  a  sensitive  plate 
will  be  reproduced,  but  if  the  size  of  the  image  be  equal  to 
the  picture,  the  second  picture  will  be  fainter  than  the  first, 
and  so  on.  Thus  again,  a  photograph  taken  on  a  dull  day 
cannot,  by  being  placed  in  bright  sunshine  be  made  to  repro- 
duce a  second  photograph  of  the  same  size  and  more  distinct- 
ly marked  than  itself;  I  at  least  have  never  succeeded  in 
such  reproduction,  and  I  am  not  aware  that  others  have  :  the 
image  loses  in  intensity  as  light  itself  does  by  each  transmis- 
sion. The  surface  of  the  metal  or  paper  may  give  a  brighter 
image  from  its  being  exposed  to  a  more  intense  light,  but  the 
photographic  details  are  limited  to  the  intensity  of  the  first 
impression,  or  rather  to  something  short  of  this.  A  question 
of  theoretical  interest  arises  from  the  consideration  of  these 
reproduced  photographs.  We  know  that  the  luminosity  of 
the  image  at  the  focus  of  a  telescope  is  limited  by  the  area 
of  the  object-glass.  The  image  of  any  given  object  cannot 
be  intensified  by  throwing  upon  it  extraneous  light ;  it  is  in- 
deed diminished  in  intensity,  and  when  for  certain  purposes 
astronomers  illuminate  the  fields  of  their  telescopes,  they  are 
obliged  to  be  contented  with  a  loss  of  intensity  in  the  telescopic 
image. 

Now,  let  us  suppose  that  the  minutest  details  in  the  image 
of  an  object  seen  in  a  given  telescope,  and  with  a  given  pow- 
er, are  noted ;  that  then  a  photographic  plate  is  placed  in  the 
focus  of  the  same  telescope  so  as  to  obtain  a  permanent  im- 
pression of  the  image  which  has  been  viewed  by  the  eye-glass. 
Could  the  observer,  by  throwing  a  beam  of  condensed  light 
upon  the  photograph,  enable  himself  to  bring  out  fresh  details  ? 
or  in  other  words,  could  he  use  with  advantage  a  higher  pow- 
er applied  to  the  illuminated  photograph  ? 

It  is,  perhaps,  hardly  safe  to  answer  a  priori  this  question  ; 
but  the  experiment  of  reproducing  photographs  would  seem  to 
show  that  more  than  the  initial  light  cannot  be  got,  and  that  we 
cannot  expect  to  increase  telescopic  power  by  photography, 


LIGHT.  115 

though  we  may  render  observations  more  convenient ;  may 
by  its  means  fix  images  seen  on  rare  and  favourable  occasions, 
and  may  preserve  permanent  and  infallible  records  of  the 
past  state  of  astronomical  objects. 

The  eifect  of  light  on  chemical  compounds  affords  us  a 
striking  instance  of  the  extent  to  which  a  force,  ever  active, 
may  be  ignored  through  successive  ages  of  philosophy.  If 
we  suppose  the  walls  of  a  large  room  covered  with  photo- 
graphic apparatus,  the  small  amount  of  light  reflected  from 
the  face  of  a  person  situated  in  its  centre  would  simulta- 
neously imprint  his  portrait  on  a  multitude  of  recipient  sur- 
faces. Were  the  cameras  absent,  but  the  room  coated  with 
photographic  paper,  a  change  would  equally  take  place  in 
every  portion  of  it,  though  not  a  reproduction  of  form  and 
figure.  As  other  substances  not  commonly  called  photo- 
graphic are  known  to  be  affected  by  light,  the  list  of  which 
might  be  indefinitely  extended,  it  becomes  a  curious  object  of 
contemplation  to  consider  how  far  light  is  daily  operating 
changes  in  ponderable  matter — how  far  a  force,  for  a  long 
time  recognised  only  in  its  visual  effects,  may  be  constantly 
producing  changes  in  the  earth  and  atmosphere,  in  addition 
to  the  changes  it  produces  in  organised  structures  which  are 
now  beginning  to  be  extensively  studied.  Thus  every  portion 
of  light  may  be  supposed  to  write  its  own  history  by  a  change 
more  or  less  permanent  in  ponderable  matter. 

The  late  Mr.  George  Stephenson  had  a  favourite  idea, 
which  would  now  be  recognised  as  more  philosophical  than  it 
was  in  his  day,  viz.  that  the  light,  which  we  nightly  obtain 
from  coal  or  other  fuel,  was  a  reproduction  of  that  which  had 
at  one  time  been  absorbed  by  vegetable  structures  from  the 
sun.  The  conviction  that  the  transient  gleam  leaves  its  per- 
manent impress  on  the  world's  history,  also  leads  the  mind 
to  ponder  over  the  many  possible  agencies  of  which  we  of  the 
present  day  may  be  as  ignorant  as  the  ancients  were  of  the 
chemical  action  of  light. 


116  CORRELATION   OF  PHYSICAL   FORCES. 

I  have  used  the  term  light,  and  affected  by  light,  in  speak- 
ing of  photographic  effects ;  but,  though  the  phenomena  de- 
rived their  name  from  light,  it  has  been  doubted  by  many 
competent  investigators  whether  the  phenomena  of  photo- 
graphy are  not  mainly  dependent  upon  a  separate  agent  ac- 
companying light,  rather  than  upon  light  itself.  It  is,  indeed, 
difficult  not  to  believe  that  a  picture,  taken  in  the  focus  of  a 
camera-obscura,  and  which  represents  to  the  eye  all  the  gra- 
dations of  light  and  shade  shown  by  the  original  luminous 
image,  is  not  an  effect  of  light;  certain  it  is,  however,  that 
the  different  coloured  rays  exercise  different  actions  upon  va- 
rious chemical  compounds,  and  that  the  effects  on  many,  per- 
haps on  most  of  them,  are  not  proportionate  in  intensity  to 
the  effects  upon  the  visual  organs.  Those  effects,  however, 
appear  to  be  more  of  degree  than  of  specific  difference ;  and, 
without  pronouncing  myself  positively  upon  the  question, 
hitherto  so  little  examined,  I  think  it  will  be  safer  to  regard 
the  action  on  photographic  compounds  as  resulting  from  a 
function  of  light.  So  viewing  it,  we  get  light  as  an  initia- 
ting force,  capable  of  producing,  mediately  or  immediately, 
the  other  modes  of  force.  Thus,  it  immediately  produces 
chemical  action  ;  and  having  this,  we  at  once  acquire  a  means 
of  producing  the  others.  At  my  Lectures  in  1843,  I  showed 
an  experiment  by  which  the  production  of  all  the  other  modes 
of  force  by  light  is  exhibited :  I  may  here  shortly  describe  it. 
A  prepared  daguerreotype  plate  is  enclosed  in  a  box  filled 
with  water,  having  a  glass  front  with  a  shutter  over  it.  Be- 
tween this  glass  and  the  plate  is  a  gridiron  of  silver  wire ; 
the  plate  is  connected  with  one  extremity  of  a  galvanometer 
coil,  and  the  gridiron  of  wire  with  one  extremity  of  a  Bre- 
guet's  helix — an  elegant  instrument,  formed  by  a  coil  of  two 
metals,  the  unequal  expansion  of  which  indicates  slight 
changes  in  temperature — the  other  extremities  of  the  galva- 
nometer and  helix  are  connected  by  a  wire,  and  the  needles 
brought  to  zero.  As  soon  as  a  beam  of  either  daylight  or 


LIGHT.  117 

the  oxyhydrogen  light  is,  by  raising  the  shutter,  permitted  to 
impinge  upon  the  plate,  the  needles  are  deflected.  Thus, 
light  being  the  initiating  force,  we  get  chemical  action  on  the 
plate,  electricity  circulating  through  the  wires,  magnetism  in 
the  coil,  heat  in  the  helix,  and  motion  in  the  needles. 

If  two  plates  of  platinum  be .  placed  in  acidulated  water, 
and  connected  with  a  delicate  galvanometer,  the  needle  of 
this  is  always  deflected,  a  result  due  to  films  of  gas  or  other 
matter  on  the  surface  of  the  platinum,  which  no  cleaning  can 
remove.  If,  after  the  needle  has  returned  to  zero,  which  will 
not  be  the  case  for  some  hours  or  even  days,  one  of  the  plat- 
inum surfaces  be  exposed  to  light,  a  fresh  deflection  of  the 
needle  takes  place,  due,  as  far  as  I  have  been  able  to  resolve 
it,  to  an  augmentation  of  the  chemical  action  which  had  occa- 
sioned the  original  deflection,  for  the  deviation  is  in  the  same 
direction.  If,  instead  of  white  light,  coloured  light  be  per- 
mitted to  impinge  on  the  plate,  the  deviation  is  greater  with 
blue  than  with  red  or  yellow  light,  showing,  in  addition  to 
other  tests,  that  the  effect  is  not  due  to  the  heat  of  the  sun's 
rays,  as  the  calorific  effects  of  light  are  greater  with  red  than 
with  blue  light,  while  the  chemical  effects  are  the  inverse. 

There  are  other  apparently  more  direct  agencies  of  light 
in  producing  electricity  and  magnetism,  such  as  those  ob- 
served by  Morichini  and  others,  as  well  as  its  effects  upon 
crystallization  ;  but  these  results  have  hitherto  been  of  so  in- 
definite a  character,  that  they  can  only  be  regarded  as  pre- 
senting fields  for  experiment,  and  not  as  proving  the  relations 
of  light  to  the  other  forces. 

Light  would  seem  directly  to  produce  heat  in  the  phenom- 
ena of  what  is  termed  absorption  of  light :  in  these  we  find 
that  heat  is  developed  in  some  proportion  to  the  disappear- 
ance of  light.  To  take  the  old  experiment  of  placing  a  se- 
ries of  different  coloured  pieces  of  cloth  upon  snow  exposed 
to  sunshine,  the  black  cloth  absorbing  the  most  light,  and  de- 
veloping the  most  heat,  sinks  more  deeply  in  the  snow  than 


118  CORRELATION   OF   PHYSICAL   FORCES. 

any  others ;  the  other  colours  or  shades  of  colour  sink  the 
more  deeply  in  proportion  as  they  absorb  or  cause  to  disap- 
pear the  more  light,  until  we  come  to  the  white  cloth,  which 
remains  upon  the  surface.  The  heating  powers  of  different 
colours  are,  however,  not  by  any  means  in  exact  proportion 
to  the  intensity  of  their  light  as  affecting  the  visual  organs. 
Thus  red  light,  when  produced  by  refraction  from  a  prism  of 
glass,  produces  greater  heating  effect  than  yellow  light  in  the 
phenomena  of  absorption,  as  has  been  observed  by  Sir  W. 
Herschel.  The  red  rays  appear,  however,  to  produce  a  dy- 
namic effect  greater  than  any  of  the  others  ;  thus  they  pene- 
trate water  to  a  greater  depth  than  the  other  colours ;  but, 
according  to  Dr.  Seebeck,  we  get  a  further  anomaly,  viz. 
that  when  light  is  refracted  by  a  prism  of  water  the  yellow 
rays  produce  the  greater  heating  effect.  The  subject,  there- 
fore, requires  much  more  experiment  before  we  can  ascertain 
the  rationale  of  the  action  of  the  forces  of  light  and  heat  in 
this  class  of  phenomena. 

In  a  former  edition  of  this  Essay,  I  suggested  the  follow- 
ing experiment  on  this  subject : — Let  a  beam  of  light  be 
passed  through  two  plates  of  tourmaline,  or  similar  sub- 
stance, and  the  temperature  of  the  second  plate,  or  that  on 
which  the  light  last  impinges,  be  examined  by  a  delicate  ther- 
moscope,  first  when  it  is  in  a  position  to  transmit  the  polar- 
ised beam  coming  from  the  first  plate,  and  secondly  when  it 
has  been  turned  round  through  an  arc  of  90°,  and  the  polar- 
ised beam  is  absorbed.  I  expected  that,  if  the  experiment 
were  carefully  performed,  the  temperature  of  the  second  plate 
would  be  more  raised  in  the  second  case  than  in  the  first,  and 
that  it  might  afford  interesting  results  when  tried  with  light 
of  different  colours.  I  met  with  difficulties  in  procuring  a 
suitable  apparatus,  and  was  endeavouring  to  overcome  them 
when  I  found  that  Knoblauch  had,  to  some  extent,  realised 
this  result.  He  finds  that,  when  a  solar  beam,  polarised  in  a 
certain  plane,  is  transmitted  perpendicularly  to  the  axis  of  a 


LIGHT.  119 

crystal  of  brown  quartz  or  tourmaline,  the  heat  is  transmit- 
ted in  a  smaller  proportion  than  when  the  beam  passes  along 
the  direction  of  the  axis  of  the  crystal. 

It  is  generally — as  far  as  I  am  aware,  universally — true 
that,  while  light  continues  as  light,  even  though  reflected  or 
transmitted  by  different  media,  little  or  no  heat  is  developed : 
and,  as  far  as  we  can  judge,  it  would  appear  that,  if  a  me- 
dium were  perfectly  transparent,  or  if  a  surface  perfectly  re- 
flected light,  not  the  slightest  heating  effect  would  take  place  ; 
but,  wherever  light  is  absorbed,  then  heat  takes  its  place,  af- 
fording us  apparently  an  instance  of  the  conversion  of  light 
into  heat,  and  of  the  fact  that  the  force  of  light  is  not,  in  fact, 
absorbed  or  annihilated,  but  merely  changed  in  character, 
becoming  in  this  instance  converted  into  heat  by  impinging 
on  solid  matter,  as  in  the  instance  mentioned  in  treating  of 
heat,  this  force  was  shown  to  be  converted  into  light  by  im- 
pinging on  solid  matter.  As,  however,  I  have  before  ob- 
served, this  correlation  of  light  and  heat  is  not  so  distinct,  as 
with  the  other  affections  of  matter.  One  experiment,  indeed, 
of  Melloni,  already  mentioned,  would  seem  to  show  that 
light  may  exist  in  a  condition  in  which  it  does  not  produce 
heat,  which  our  instruments  are  able  to  detect ;  but  some 
doubt  has  recently  been  thrown  on  the  accuracy  of  this  ex- 
periment ;  probably  the  substances  themselves  through  which 
the  light  is  transmitted  would  be  found  to  have  been  heated. 

The  recipient  body,  or  that  upon  which  light  impinges, 
seems  to  exercise  as  important  an  influence  on  OVT  percep- 
tions of  light  as  the  emittent  body,  or  that  from  which  the 
light  first  proceeds.  The  recent  experiments  o.  Sir  John 
Herschel  and  Mr.  Stokes  show  that  radiant  imputes,  which, 
falling  on  certain  bodies,  give  no  effect  of  light,  become  lu- 
minous when  falling  on  other  bodies. 

Thus,  let  ordinary  solar  light  be  refracted  by  t  prism  (the 
best  material  for  which  is  quartz),  and  the  spectrum  received 
on  a  sheet  of  paper,  or  of  white  porcelain ;  looking  <jn  the 


120  CORRELATION   OF   PHYSICAL   FORCES. 

paper,  the  eye  detects  no  light  beyond  the  extreme  violet 
rays.  If,  therefore,  an  opaque  body  be  interposed  so  as  just 
to  cut  off  the  whole  visible  spectrum,  the  paper  would  be 
dark  or  invisible,  with  the  exception  of  some  slight  illumina- 
tion from  light  reflected  by  the  air  and  surrounding  bodies. 
Substitute  for  that  portion  of  the  paper  which  was  beyond 
the  visible  spectrum  a  piece  of  glass  tinged  by  the  oxide  of 
uranium,  and  the  glass  is  perfectly  visible  ;  so  with  a  bottle 
of  sulphate  of  quinine,  or  of  the  juice  of  horse-chestnuts,  or 
even  paper  soaked  in  these  latter  solutions.  Other  substances 
exhibit  this  effect  in  different  degrees ;  and  among  the  sub- 
stances which  have  hitherto  been  considered  perfectly  analo- 
gous as  to  their  appearance  when  illuminated,  notable  differ- 
ences are  discovered.  Thus  it  appears  that  emanations 
which  give  no  impression  of  light  to  the  eye,  when  imping- 
ing on  certain  bodies,  become  luminous  when  impinging  on 
others.  We  might  imagine  a  room  so  constructed  that  such 
emanations  alone  are  permitted  to  enter  it,  which  would  be 
dark  or  light  according  to  the  substance  with  which  the  walls 
were  coated,  though  in  full  daylight  the  respective  coatings 
of  the  walls  would  appear  equally  white  ;  or,  without  alter- 
ing the  coating  of  the  walls,  the  room  exposed  to  one  class 
of  rays,  might  be  rendered  dark  by  windows  which  would  be 
transparent  to  another  class. 

If,  instead  of  solar  light,  the  electrical  light  be  employed 
for  similar  experiments,  an  equally  striking  effect  can  actually 
be  produced.  A  design,  drawn  on  white  paper  with  a  solu- 
tion of  sulphate  of  quinine  and  tartaric  acid,  is  invisible  by 
ordinary  light,  but  appears  with  beautiful  distinctness  when 
illuminated  by  the  electric  light.  Thus,  in  pronouncing  upon 
a  luminous  effect,  regard  must  be  had  to  the  recipient  as  well 
as  to  the  emittent  body.  That  which  is,  or  becomes,  light 
when  it  fells  upon  one  body  is  not  light  when  it  falls  upon 
another.  Probably  the  retinaB  of  the  eyes  of  different  per- 
sons differ  to  some  extent  in  a  similar  manner ;  and  the  same 


LIGHT.  121 

substance,  illuminated  by  the  same  spectrum,  may  present 
different  appearances  to  different  persons,  the  spectrum  ap- 
pearing more  elongated  to  the  one  than  to  the  other,  so  that 
what  is  light  to  the  one  is  darkness  to  the  other.  A  depend- 
ence on  the  recipient  body  may  also,  to  a  great  extent,  be 
predicated  of  heat.  Let  two  vessels  of  water,  the  contents 
of  the  one  clear  and  transparent,  of  the  other  tinged  by  some 
colouring  matter,  be  suspended  in  a  summer's  sun  ;  in  a  very 
short  time  a  notable  difference  of  temperature  will  be  ob- 
served, the  coloured  having  become  much  hotter  than  the 
clear  liquid.  If  the  first  vessel  be  placed  at  a  considerable 
distance  from  the  surface  of  the  earth,  and  the  second  near 
the  surface,  the  difference  is  still  more  considerable.  Carry- 
ing on  this  experiment,  and  suspending  the  first  over  the  top 
of  a  high  mountain,  and  the  second  in  a  valley,  we  may  ob 
tain  so  great  a  difference  of  temperature,  that  animals  whose 
organization  is  suited  for  the  one  temperature  could  not  live 
in  the  other,  and  yet  both  are  exposed  to  the  same  luminous 
rays  at  the  same  time,  and  substantially  at  the  same  distance 
from  the  emittent  body — the  substance  nearer  the  sun  is  in 
fact  colder  than  the  more  remote.  So,  with  regard  to  the 
medium  transmitting  the  influence  :  a  green-house  may  have 
its  temperature  considerably  varied  by  changing  the  glass  of 
which  its  roof  is  made. 

These  effects  have  an  important  bearing  on  certain  cos- 
mical  questions  which  have  lately  been  much  discussed,  and 
should  induce  the  greatest  caution  in  forming  opinions  on 
such  subjects  as  "light  and  heat  on  the  sun's  surface,  the  tem- 
perature of  the  planets,  &c.  This  may  depend  as  much  upon 
their  physical  constitution  as  upon  their  distance  from  the 
sun.  Indeed,  the  planet  Mars  gives  us  a  highly  probable  ar- 
gument for  this ;  for,  notwithstanding  that  it  is  half  as  far 
again  from  the  sun  as  the  earth  is,  the  increase  of  the  white 
tracts  at  its  poles  during  its  winter,  and  their 'diminution  dur- 
ing its  summer,  show  that  the  temperature  of  the  surface  of 
6 


122  CORRELATION   OF   PHYSICAL  FORCES. 

this  planet  oscillates  about  that  of  the  freezing  point  of  water, 
as  do  the  analogous  zones  of  our  planet.  It  is  true,  in  this 
we  assume  that  the  substance  thus  changing  its  state  is  water, 
but,  considering  the  many  close  analogies  of  this  planet  with 
the  earth,  and  the  identity  in  appearance  of  these  very  effects 
with  what  takes  place  on  the  earth,  it  seems  a  highly  proba- 
ble assumption. 

So  it  by  no  means  necessarily  follows,  that  because  Venus 
is  nearer  to  the  sun  than  the  earth,  that  planet  is  hotter  than 
our  globe.  The  force  emitted  by  the  sun  may  take  a  differ- 
ent character  at  the  surface  of  each  different  planet,  and 
require  different  organisms  or  senses  for  its  appreciation. 
Myriads  of  organised  beings  may  exist  imperceptible  to  our 
vision,  even  if  we  were  among  them ;  and  we  might  be  also 
imperceptible  to  them ! 

However  vain  it  may  be,  in  the  present  state  of  science, 
to  speculate  upon  such  existences,  it  is  equally  vain  to  assume 
identity  or  close  approximations  to  our  own  forms  in  those 
beings  which  may  people  other  worlds.  From  analogical 
reasoning,  or  from  final  causation,  if  that  be  admitted,  we 
may  feel  convinced  that  the  gorgeous  globes  of  the  universe 
are  not  unpeopled  deserts  ;  but  whether  the  denizens  of  other 
worlds  are  more  or  less  powerful,  more  or  less  intelligent, 
whether  they  have  attributes  of  a  higher  or  lower  class  than 
ourselves,  is  at  present  an  utterly  hopeless  guessing. 

Specific  gravity  and  intelligence  have  no  necessary  con- 
nexion. On  our  own  planet  five  senses,  and  a  mean  density 
equal  to  that  of  water,  are  not  invariably  associated  with  in- 
tellectual or  moral  greatness,  and  the  many  arguments  which 
have  been  used  to  prove  that  suns  and  planets  other  than  the 
earth  are  uninhabited,  or  not  inhabited  by  intellectual  beings, 
might,  mutatis  mutandis,  equally  be  used  by  the  denizens  of 
a  sun  or  planet  to  prove  that  this  world  was  uninhabited. 

Men  are  too  apt,  because  they  are  men,  because  their 
existence  is  the  one  thing  of  all  importance  to  themselves,  to 


LIGHT.  123 

frame  schemes  of  the  universe  as  though  it  was  formed  for 
man  alone  :  painted  by  an  artist  of  the  sun,  a  man  might  not 
represent  so  prominent  an  object  of  creation  as  he  does 
when  represented  by  his  own  pencil. 

Light  was  regarded,  by  what  was  termed  the  corpuscular 
theory,  as  being  in  itself  matter  or  a  specific  fluid  emanating 
from  luminous  bodies,  and  producing  the  effects  of  sensation 
by  impinging  on  the  retina.  This  theory  gave  way  to  the  un- 
dulatory  one,  which  is  generally  adopted  in  the  present  day, 
and  which  regards  light  as  resulting  from  the  undulation  of  a 
specific  fluid  to  which  the  name  of  ether  has  been  given, 
which  hypothetic  fluid  is  supposed  to  pervade  the  universe, 
and  to  penetrate  the  pores  of  all  bodies. 

In  a  Lecture  delivered  in  January  1842,  when  I  first 
publicly  advanced  the  views  advocated  in  this  Essay,  I  stated 
that  it  appeared  to  me  more  consistent  with  known  facts  to 
regard  light  as  resulting  from  a  vibration  or  motion  of  the 
molecules  of  matter  itself,  rather  than  from  a  specific  ether 
pervading  it ;  jnst  as  sound  is  propagated  by  the  vibrations 
of  wood,  or  as  waves  are  by  water.  I  am  not  here  speaking 
of  the  character  of  the  vibrations  of  light,  sound,  or  water, 
which  are  doubtless  very  different  from  each  other,  but  am 
only  comparing  them  so  far  as  they  illustrate  the  propagation 
of  force  by  motion  in  the  matter  itself. 

I  was  not  aware,  at  the  time  that  I  first  adopted  the 
above  view,  and  brought  it  forward  in  my  Lectures,  that  the 
celebrated  Leonard  Euler  had  published  a  somewhat  similar 
theory ;  and,  though  I  suggested  it  without  knowing  that  it 
had  been  previously  advanced,  I  should  have  hesitated  in 
reproducing  it  had  I  not  found  that  it  was  sanctioned  by  so 
eminent  a  mathematician  as  Euler,  who  cannot  be  supposed 
to  have  overlooked  any  irresistible  argument  against  it — the 
more  so  in  a  matter  so  much  controverted  and  discussed  as 
the  undulatory  theory  of  light  was  in  his  time. 

Although  this  theory  has  been  considered  defective  by  a 


124  CORRELATION  OF   PHYSICAL   FORCES. 

philosopher  of  high  repute,  I  cannot  see  the  force  of  the 
arguments  by  which  it  has  been  assailed ;  and  therefore,  for 
the  present,  though  with  diffidence,  I  still  adhere  to  it.  The 
fact  itself  of  the  correlation  of  the  different  modes  of  force  is 
to  my  mind  a  very  cogent  argument  in  favour  of  their  being 
affections  of  the  same  matter  ;  and  though  electricity,  magnet- 
ism, and  heat  might  be  viewed  as  produced  by  undulations  of 
the  same  ether  as  that  by  means  of  which  light  is  supposed  to 
be  produced,  yet  this  hypothesis  offers  greater  difficulties  with 
regard  to  the  other  affections  than  with  regard  to  light :  many 
of  these  difficulties  I  have  already  alluded  to  when  treating  of 
electricity ;  thus  conduction  and  non-conduction  are  not  ex- 
plained by  it ;  the  transmission  of  electricity  through  long 
wires  in  preference  to  the  air  which  surrounds  them,  and 
which  must  be  at  least  equally  pervaded  by  the  ether,  ig 
irreconcilable  with  such  an  hypothesis.  The  phenomena  ex- 
hibited by  these  forces  afford,  as  I  think,  equally  strong  evi- 
dence with  those  of  light,  of  ordinary  matter  acting  from  par- 
ticle to  particle,  and  having  no  action  at  a  distance.  I  have 
already  instanced  the  experiments  of  Faraday  on  electrical 
induction,  showing  it  to  be  an  action  of  contiguous  particles, 
which  are  strongly  in  favour  of  this  view,  and  many  experi- 
ments which  I  have  made  on  the  voltaic  arc,  some  of  which 
I  have  mentioned  in  this  Essay,  are,  to  my  mind,  confirma- 
tory of  it. 

If  it  be  admitted  that  one  of  the  so-called  imponderables  is 
a  mode  of  motion,  then  the  fact  of  its  being  able  to  produce 
the  others,  and  be  produced  by  them,  renders  it  highly  diffi- 
cult to  conceive  some  as  molecular  motions  and  others  as 
fluids  or  undulations  of  an  ether.  To  the  main  objection  of 
Dr.  Young,  that  all  bodies  would  have  the  properties  of  solar 
phosphorus  if  light  consisted  in  the  undulations  of  ordinary 
matter,  it  may  be  answered  that  so  many  bodies  have  this 
property,  and  with  so  great  a  variety  in  its  duration,  that 
non  constat  all  may  not  have  it,  though  for  a  time  so  short 


LIGHT.  125 

that  the  eye  cannot  detect  its  duration.  M.  E.  Becquerel 
has  made  many  experiments  which  support  this  view ;  the 
fact  of  the  phosphorescence  by  insolation  of  a  large  number 
of  bodies,  is  in  itself  evidence  of  the  matter  of  which  they  are 
composed  being  thrown  into  a  state  of  undulation,  or  at  all 
events  molecularly  affected  by  the  impact  of  light,  and  is 
therefore  an  argument  in  support  of  the  view  to  which  objec- 
tion is  taken.  Dr.  Young  admits  that  the  phenomena  of 
solar  phosphorus  appear  to  resemble  greatly  the  sympathetic 
sounds  of  musical  instruments,  which  are  agitated  by  other 
sounds  conveyed  to  them  through  the  air,  and  I  am  not  aware 
that  he  gives  any  explanation  of  these  effects  on  the  ethereal 
hypothesis. 

Some  curious  experiments  of  M.  Niepce  de  St.  Victor 
seem  also  to  present  an  analogy  in  luminous  phenomena  to 
sympathetic  sounds.  An  engraving  which  has  been  kept  for 
some  days  in  the  dark  is  half  covered  by  an  opaque  screen, 
and  then  exposed  to  the  sun ;  it  is  then  removed  from  the 
light,  the  screen  taken  away,  and  the  engraving  placed  oppo- 
site, and  at  a  short  distance  from,  photographic  paper :  an 
inverted  image  of  that  portion  of  the  engraving  which  has 
been  exposed  to  the  sun  is  produced  on  the  photographic 
paper,  while  the  part  which  had  been  covered  by  the  screen 
is  not  reproduced.  If  the  engraving,  after  exposure,  is 
allowed  to  remain  in  contact  with  white  paper  for  some  hours, 
and  the  white  paper  is  then  placed  upon  photographic  paper, 
a'  faint  image  of  the  exposed  portion  of  the  engraving  is  repro- 
duced. Similar  results  are  produced  by  mottled  marble  ex- 
posed to  the  sun  ;  an  invisible  tracing  on  paper  by  a  fluores- 
cent body,  sulphate  of  quinine,  is,  after  insolation,  reproduced 
on  the  photographic  paper.  Insolated  paper  retains  the  power 
of  producing  an  impression  for  a  very  long  period,  if  it  is  kept 
in  an  opaque  tube  hermetically  closed. 

It  is  right  to  observe  that  these  effects  are  supposed  by 
many  to  be  due  to  chemical  emanations  proceeding  from  the 


126  CORRELATION   OF   PHYSICAL   FOECES. 

substances  exposed  to  the  sun,  and  which  are  believed  to  have 
undergone  some  chemical  change  by  this  exposure.  It  is 
desirable  to  await  further  experiment  before  forming  a  decid- 
ed opinion. 

The  analogies  in  *the  progression  of  sound  and  light  are 
very  numerous :  each  proceed  in  straight  lines,  until  inter- 
rupted ;  each  is  reflected  in  the  same  manner,  the  angles  of 
incidence  and  reflexion  being  equal ;  each  is  alternately  nulli- 
fied and  doubled  in  intensity  by  interference  ;  each  is  capable 
of  refraction  when  passing  from  media  of  different  density : 
this  last  effect  of  sound,  long  ago  theoretically  determined, 
has  been  experimentally  proved  by  Mr.  Sondhauss,  who  con- 
structed a  lens  of  films  of  collodion,  which,  when  filled  with 
carbonic  acid,  enabled  him  to  hear  the  ticking  of  a  watch 
placed  in  one  focus  of  the  lens,  the  ear  of  the  experimenter 
being  in  the  opposite  focus.  The  ticking  was  not  heard 
when  the  watch  was  moved  aside  from  the  focal  point,  though 
it  remained  at  an  equal  distance  from  the  ear.  An  experi- 
ment of  M.  Dove  seems,  indeed,  to  show  an  effect  of  polari- 
sation of  sound. 

The  phenomena  presented  by  heat,  viewed  according  to 
the  dynamic  theory,  cannot  be  explained  by  the  motion  of  an 
imponderable  ether,  but  involve  the  molecular  actions  of 
ordinary  ponderable  matter.  The  doctrine  of  propagation  by 
undulations  of  ordinary  matter  is  very  generally  admitted  by 
those  who  support  the  dynamical  theory  of  heat ;  but  the 
analogies  of  the  phenomena  presented  by  heat  and  light  are 
so  close,  that  I  cannot  see  how  a  theory  applied  to  the  one 
agent  should  not  be  applicable  to  the  other.  When  heat  is 
transmitted,  reflected,  refracted,  or  polarised,  can  we  view 
that  as  an  affection  of  ordinary  matter,  and  when  the  same 
effects  take  place  with  light,  view  the  phenomena  as  pro- 
duced by  an  imponderable  ether,  and  by  that  alone  ? 

An  objection  that  immediately  occurs  to  the  mind  in 
reference  to  the  ethereal  hypothesis  of  light  is,  that  the  most 


LIGHT.  127 

porous  bodies  are  opaque  ;  cork,  charcoal,  pumice  stone,  dried 
and  moist  wood,  &c.,  all  very  porous  and  very  light,  are  all 
opaque.  This  objection  is  not  so  superficial  as  it  might  seem 
at  first  sight.  The  theory  which  assumes  that  light  is  an 
undulation  of  an  ethereal  medium  pervading  gross  matter, 
assumes  the  distances  between  the  molecules  or  atoms  of 
matter  to  be  very  great.  Matter  has  been  likened  by  Demo- 
critus,  and  by  many  modern  philosophers,  to  the  starry  firma- 
ment, in  which,  though  the  individual  monads  are  at  immense 
distances  from  each  other,  yet  they  have  in  the  aggregate  a 
character  of  unity,  and  are  firmly  held  by  attraction  in  their 
respective  positions  and  at  definite  distances.  Now,  if  matter 
be  built  up  of  separate  molecules,  then,  as  far  as  our  knowl- 
edge extends,  the  lightest  bodies  would  be  those  in  which  the 
molecules  are  at  the  greatest  distances,  and  those  in  which 
any  undulation  of  a  pervading  medium  would  be  the  least 
interfered  with  by  the  separated  particles — such  bodies  should 
consequently  be  the  most  transparent. 

If,  again,  the  analogy  of  the  starry  firmament  held  good, 
in  this  case  an  undulation  or  wave  proportioned  to  the  indivi- 
dual monads  would  be  broken  up  by  the  number  of  them,  and 
the  very  appearance  of  continuity  which  results,  as  in  the 
milky  way,  from  each  point  of  vision  being  occupied  by  one 
of  the  monads,  would  show  that  at  some  portion  of  its  pro- 
gress the  wave  is  interrupted  by  one  of  them,  so  that  the 
whole  may  be  viewed  in  some  respect  as  a  sheet  of  ordinary 
matter  interposed  in  the  ethereal  expanse. 

Even  then,  if  it  be  admitted  that  a  highly  elastic  medium 
pervades  the  interspaces,  the  separate  masses  as  a  whole  must 
exercise  an  important  influence  on  the  progress  of  the  wave. 

Sound  or  vibrations  of  air  meeting  with  a  screen,  or,  as 
it  were,  sponge  of  diffused  particles,  would  be  broken  up  and 
dispersed  by  them  ;  but  if  they  be  sufficiently  continuous  to 
take  up  the  vibration  and  propagate  it  themselves,  the  sound 
continues  comparatively  unimpaired.  % 


128  COKKELATION   OF   PHYSICAL  FORCES. 

With  regard,  however,  to  liquid  and  gaseous  bodies,  there 
are  very  great  difficulties  in  viewing  them  as  consisting  of 
separate  and  distant  molecules.  If,  for  instance,  we  assume 
with  Young  that  the  particles  in  water  are  at  least  as  distant 
from  each  other  comparatively  as  100  men  would  be  if  dis- 
persed at  equal  distances  over  the  surface  of  England,  the  dis- 
tance of  these  particles,  when  the  water  is  expanded  into 
steam,  would  be  increased  more  than  forty  times,  so  that  the 
100  men  would  be  reduced  to  two,  and  by  further  increasing 
the  temperature  this  distance  may  be  indefinitely  increased  ; 
adding  to  the  effects  of  temperature  rarefaction  by  the  air- 
pump,  we  may  again  increase  the.  distance,  so  that,  if  we  as- 
sume any  original  distance,  we  ought,  by  expansion,  to  in- 
crease it  to  a  point  at  which  the  distance  between  molecule 
and  molecule  should  become  measurable.  But  no  extent  of 
rarefaction,  whether  by  heat  or  the  air-pump,  or  both,  makes 
the  slightest  change  in  the  apparent  continuity  of  matter ; 
and  gases,  I  find,  retain  their  peculiar  character,  as  far  as  a 
judgment  of  it  can  be  formed  from  its  effect  on  the  electric 
spark,  throughout  any  extent  of  rarefaction  which  can  exper- 
imentally be  applied  to  them  :  thus  the  electric  spark  in  prot- 
oxide of  nitrogen,  however  attenuated,  presents  a  crimson 
tint,  that  in  carbonic  oxide  a  greenish  tint. 

"Without,  however,  entering  on  the  metaphysical  enquiry 
as  to  the  constitution  of  matter  (or  whether  the  atomic  phil- 
osophers or  the  followers  of  Boscovich  are  right) ,  a  question 
which  probably  human  appliances  will  never  answer :  and 
even  admitting  that  an  ethereal  medium,  not  absolutely  im- 
ponderable as  asserted  by  many,  but  of  extreme  tenuity,  per- 
vades matter,  still  ordinary  or  non-ethereal  matter  itself  must 
exercise  a  most  important  action  upon  the  transmission  of 
light ;  and  Dr.  Young,  who  opposed  the  theory  of  Euler,  that 
light  was  transmitted  by  undulations  of  gross  matter  itself, 
just  as  sound  is,  was  afterwards  obliged  to  call  to  his  assis- 
tance the  vibrations  of  the  ponderable  matter  of  the  refract- 


LIGHT.  129 

ing  media,  to  explain  why  rays  of  all  colours  were  not  equal- 
ly refracted,  and  other  difficulties.  One  of  his  arguments  in 
support  of  the  existence  of  a  permeating  ether  was,  "  that  a 
medium  resembling  in  many  properties  that  which  has  been 
denominated  Ether  does  exist,  is  undeniably  proved  by  the 
phenomena  of  electricity."  This  seems  to  me,  if  I  may  ven- 
ture to  say  so  of  anything  proceeding  from  so  eminent  a  man, 
scarcely  logical :  ilf is  supporting  one  hypothesis  by  another, 
and  considering  that  to  be  proved  which  its  most  strenuous 
advocates  admit  to  be  surrounded  by  very  many  difficulties. 

If  it  be  said  that  there  is  not  sufficient  elasticity  in  ordi- 
nary matter  for  the  transmission  of  undulations  with  such  ve- 
locity as  light  is  known  to  travel,  this  may  be  so  if  the  vibra- 
tions be  supposed  exactly  analogous  to  those  of  sound  ;  but 
that  molecular  motion  can  travel  with  equal  and  even  greater 
velocity  than  light,  is  shown  by  the  rapidity  with  which  elec- 
tricity traverses  a  metal  wire  where  each  particle  of  metal  is 
undoubtedly  affected.  It  has,  moreover,  been  shown  by  the 
experiments  of  Mr.  Latimer  Clarke  upon  a  length  of  wire 
of  760  miles,  that  whatever  be  the  intensity  of  electrical  cur- 
rents, they  are  propagated  with  the  same  velocity  provided 
the  effects  of  lateral  induction  be  the  same — a  striking  anal- 
ogy with  one  of  the  effects  observed  in  the  propagation  of 
light  and  sound.  The  effects  observed  by  MM.  Fizeau  and 
Foucault,  of  the  slower  progression  of  light  in  proportion  as 
the  transmitting  medium  is  more  dense,  seem  to  me  in  favour 
of  the  view  here  advocated ;  as  a  greater  degree  of  heat 
would  be  produced  by  light  in  proportion  to  the  density  of 
the  medium,  force  would  be  thus  carried  off,  and  the  molecular 
system  disturbed  so  that  the  progress  of  the  motion  should  be 
more  slow  ;  but  so  many  considerations  enter  into  this  question, 
and  the  phenomena  are  so  extremely  complex,  that  it  would 
be  rash  to  hazard  any  positive  opinion. 

Dr.  Young  ultimately  came  to  the  conclusion  that  it  was 
simplest  to  consider  the  ethereal  medium,  together  with  the 
6* 


130  CORRELATION   OF  PHYSICAL   FORCES. 

material  atoms  of  the  substance,  as  constituting  together  a 
compound  medium  denser  than  pure  ether,  but  not  more  elas- 
tic. Ether  might  thus  be  viewed  as  performing  the  functions 
which  oil  does  with  tracing  paper,  giving  continuity  to  the 
particles  of  gross  matter,  and  in  the  interplanetary  spaces 
forming  itself  the  medium  which  transmits  the  undulations. 

Since  the  period  when  Huyghens,  Euler,  and  Young,  the 
fathers  of  the  undulatory  theory,  applied  their  great  minds  to 
this  subject,  a  mass  of  experimental  data  has  accumulated, 
all  tending  to  establish  the  propositions,  that  whenever  matter 
transmitting  or  reflecting  light  undergoes  a  structural  change, 
the  light  itself  is  affected,  and  that  there  is  a  connection  or 
parallelism  between  the  change  in  the  matter  and  the  change 
in  the  affection  of  light,  and  conversely  that  light  will  modify 
or  change  the  structure  of  matter  and  impress  its  molecules 
with  new  characteristics. 

Transparency,  opacity,  refraction,  reflection,  and  colour 
were  phenomena  known  to  the  ancients,  but  sufficient  attention 
does  not  appear  to  have  been  paid  by  them  to  the  molecular 
states  of  the  bodies  producing  these  effects  ;  thus  the  trans- 
parency or  opacity  of  a  body  appears  to  depend  entirely  upon 
its  molecular  arrangement.  If  stride  occur  in  a  lens  or  glass 
through  which  objects  are  viewed,  the  objects  are  distorted  : 
increase  the  number  of  these  striae,  the  distortion  is  so  in- 
creased that  the  objects  become  invisible,  and  the  glass  ceases 
to  be  transparent,  though  remaining  translucent ;  but  alter 
completely  the  molecular  structure,  as  by  slow  solidification, 
and  it  becomes  opaque.  Take,  again,  an  example  of  a  liquid 
and  a  gas :  a  solution  of  soap  is  transparent,  air  is  transpar- 
ent, but  agitate  them  together  so  as  to  form  a  froth  or  lather, 
and  this,  though  consisting  of  two  transparent  bodies,  is 
opaque  ;  and  the  reflection  of  light  from  the  surface  of  these 
bodies,  when  so  intermixed,  is  strikingly  different  from  its  re- 
flection before  mixture,  in  the  one  case  giving  to  the  eye  a 
mere  general  effect  of  whiteness,  in  the  other  the  images  of 
objects  in  their  proper  shapes  and  colours. 


LIGHT.  131 

To  take  a  more  refined  instance :  nitrogen  is  perfectly 
colourless,  oxygen  is  perfectly  colourless,  but  chemically  uni- 
ted in  certain  proportions  they  form  nitrous  acid,  a  gas  which 
has  a  deep  orange  brown  colour.  I  know  not  how  the  col- 
our of  this  gas,  or  of  such  gases  as  chlorine  or  vapour  of 
iodine,  can  be  accounted  for  by  the  ethereal  hypothesis,  with- 
out calling  in  aid  molecular  affections  of  the  matter  of  these 
gases. 

Colour  in  many  instances  depends  upon  the  thickness  of 
the  plate  or  film  of  transparent  matter  upon  which  light  is  in- 
cident ;  as  in  all  those  cases  which  are  termed  the  colours  of 
thin  plates,  of  which  the  soap  bubble  affords  a  beautiful  in- 
stance. 

When  we  arrive  at  the  more  recent  discoveries  of  double 
refraction  and  polarisation,  the  effects  of  light  are  found  to 
trace  out  as  it  were  the  structure  of  the  matter  affected,  and 
the  crystalline  form  of  a  body  can  be  determined  by  the 
effects  which  a  minute  portion  of  it  exercises  on  a  ray  of 
light. 

Let  a  piece  of  good  glass  be  placed  in  what  is  called  a 
polariscope,  or  instrument  in  which  light  that  has  undergone 
polarisation  is  transmitted  through  the  substance  to  be  exam- 
ined, and  the  emergent  light  is  afterwards  submitted  to  anoth- 
er substance  capable  of  polarising  light,  or,  as  it  is  termed,  an 
analyser  ;  no  change  in  effect  will  be  observed.  Remove  the 
glass,  heat  it  and  suddenly  or  quickly  cool  it  as  to  render  it 
unannealed,  in  which  state  its  molecules  are  in  a  state  of 
tension  or  strain,  and  the  glass  highly  brittle,  on  replacing  it 
in  the  polariscope,  a  beautiful  series  of  colours  is  perceptible. 
Instead  of  subjecting  the  glass  to  heat  and  sudden  cooling, 
let  it  be  bent  or  strained  by  mechanical  pressure,  and  the  col- 
ours will  be  equally  visible,  modified,  according  to  the  direc- 
tion of  the  flexure,  and  indicating  by  their  course  the  curves 
where  the  molecular  state  has  been  changed  by  pressure.  So 
if  tough  glue  be  elongated  and  allowed  to  cool  in  a  stretched 


132  COKKELATION   OF   PHYSICAL   FOECES. 

state,  it  doubly  refracts  light,  and  the  colours  are  shown  as  in 
the  instance  of  glass. 

Submit  a  series  of  crystals  to  the  same  examination,  and 
different  figures  will  be  formed  by  different  crystals,  bearing 
a  constant  and  definite  relation  to  the  structure  of  the  partic- 
ular crystal  examined,  and  to  the  direction  in  which,  with 
reference  to  crystalline  form,  the  ray  crosses  the  crystal. 

In  the  crystallised  salts  of  paratartaric  acid,  M.  Pasteur 
noticed  two  'sets  of  crystals  which  were  hemihedral  in  oppo- 
site directions,  i.  e.  the  crystals  of  one  set  were  to  those  of 
the  other  as  to  their  own  image  reflected  in  a  mirror ;  on 
making  a  separate  solution  of  each  of  these  classes  of  crys- 
tals, he  found  that  the  solution  of  the  one  class  rotated  the 
plane  of  polarisation  to  the  right,  while  that  of  the  other 
class  rotated  to  the  left,  and  that  a  mixture  in  proper  propor- 
tions of  the  two  solutions  produced  no  deviation  in  the  plane 
of  polarisation.  Yet  all  these  three  solutions  are  what  is  term- 
ed isomeric,  that  is,  have  as  far  as  can  be  discovered  the  same 
chemical  constitution. 

In  the  above,  and  in  innumerable  other  cases,  it  is  seen 
that  an  alteration  in  the  structure  of  a  transparent  substance 
alters  the  character  and  effects  of  the  transmitted  light.  The 
phenomena  of  photography  prove  that  light  alters  the  struc- 
ture of  matter  submitted  to  it ;  with  regard  even  to  vision  it- 
self, the  persistence  of  images  on  the  retina  of  the  eye  would 
seem  to  show  that  its  structure  was  changed  by  the  impact 
of  light,  the  luminous  impressions  being  as  it  were  branded 
on  the  retina,  and  the  memory  of  the  vision  being  the  scar  of 
such  brand.  The  science  of  photography  has  reference  main- 
ly to  solid  substances,  yet  there  are  many  instances  of  liquid 
and  gaseous  bodies  being  changed  by  the  action  of  light :  thus 
hydrocyanic  acid,  a  liquid,  undergoes  a  chemical  change  and 
deposits  a  solid  carbonaceous  compound  by  the  action  of 
light.  Chlorine  and  hydrogen  gases,  when  mixed  and  pre- 
served in  darkness,  do  not  unite,  but  when  exposed  to  light 
rapidly  combine,  forming  hydrochloric  acid. 


LIGHT.  133 

The  above  facts — and  many  others  might  have  been  given 
— go  far  to  connect  light  with  motion  of  ordinary  matter,  and 
to  show  that  many  of  the  evidences  which  our  senses  receive 
of  the  existence  of  light  result  from  changes  in  matter  itself. 
When  the  matter  is  in  the  solid  state,  these  changes  are  more 
or  less  permanent ;  when  in  the  liquid  or  gaseous  state,  they 
are  temporary  in  the  greater  number  of  instances,  unless  there 
be  some  chemical  change  effected,  which  is,  as  it  were,  seized 
upon  during  its  occurrence,  and  a  resulting  compound  formed, 
which  is  more  stable  than  the  original  compound  or  mix- 
ture. 

I  might  weary  my  reader  with  examples, "  showing  that, 
in  every  case  which  we  can  trace  out,  the  effects  of  light  are 
changed  by  any  and  every  change  of  structure,  and  that  light 
has  a  definite  connection  with  the  structure  of  the .  bodies 
affected  by  it.  I  cannot  but  think  that  it  is  a  strong  assump- 
tion to  regard  ether,  a  purely  hypothetical  creation,  as  chang- 
ing its  elasticity  for  each  change  of  structure,  and  to  regard 
it  as  penetrating  the  pores  of  bodies  of  whose  porosity  we 
have  in  many  cases  no  proof;  the  which  pores  must,  more- 
over, have  a  definite  and  peculiar  communication,  also  assumed 
for  the  purpose  of  the  theory. 

Ether  is  a  most  convenient  medium  for  hypothesis  :  thus, 
if  to  account  for  a  given  phenomenon  the  hypothesis  requires 
that  the  ether  be  more  elastic,  it  is  said  to  be  more  elastic  ; 
if  more  dense,  it  is  said  to  be  more  dense  ;  if  it  be  required 
by  hypothesis  to  be  less  elastic,  it  is  pronounced  to  be  less 
elastic  ;  and  so  on. 

The  advocates  of  the  ethereal  hypothesis  certainly  have 
this  advantage,  that  the  ether,  being  hypothetical,  can  have 
its  characters  modified  or  changed  without  any  possibility  of 
disproof  either  of  its  existence  or  modifications. 

It  may  be  that  the  refined  mathematical  labours  on 
light,  as  on  electricity,  have  given  an  undue  and  adventitious 
strength  to  the  hypotheses  on  which  they  are  based. 


134  CORRELATION   OF  PHYSICAL   FORCES, 

An  objection  to  which  the  view  I  have  been  advocating  is 
open,  and  a  formidable  one,  is,  the  necessity  involved  in  it  of 
an  universal  plenum ;  for  if  light,  heat,  electricity,  &c.,  be  affec- 
tions of  ordinary  matter,  then  matter  must  be  supposed  to  be 
everywhere  where  these  phenomena  are  apparent,  and  con- 
sequently there  can  be  no  vacuum. 

These  forces  are  transmitted  through  what  are  called 
vacua,  or  through  the  interplanetary  spaces,  where  matter,  if 
it  exist,  must  be  in  a  highly  attenuated  state. 

It  may  be  safely  stated  that  hitherto  all  attempts  at  pro- 
curing a  perfect  vacuum  have  failed.  The  ordinary  air- 
pump  gives  us  only  highly  rarefied  air  ;  and,  by  the  principle 
of  construction,  even  of  the  best,  the  operation  depends  upon 
the  indefinite  expansion  of  the  volume  of  air  in  the  receiver  ; 
even  in  the  vacuum  which  is  formed  in  this,  so  great  is  the 
tendency  of  matter  to  fill  up  space,  that  I  have  observed  dis- 
tilled water  contained  in  a  vessel  within  the  exhausted  receiv- 
er of  a  good  air-pump  has  a  taste  of  tallow,  derived  from  the 
grease,  or  an  essential  oil  contained  in  it,  which  is  used  to 
form  an  air-tight  junction  between  the  edges  of  the  receiver 
and  the  pump-plate. 

The  Torricellian  vacuum,  or  that  of  the  ordinary  baro- 
meter, is  filled  with  the  vapour  of  mercury  ;  but  it  might  be 
worth  the  trouble  to  ascertain  what  would  be  the  effect  of  a 
good  Torricellian  vacuum,  when  the  mercury  in  the  tube  is 
frozen,  which  might,  without  much  difficulty,  be  now  effected 
by  the  use  of  solid  carbonic  acid  and  ether ;  the  only  proba- 
ble difficulty  would  be  the  different  rates  of  contraction  of 
mercury  and  glass,  at  such  a  degree  of  cold,  and  more  par- 
ticularly the  contraction  of  mercury  at  the  period  of  its 
solidification.  Davy,  however,  endeavoured  to  form  a 
vacuum,  in  a  somewhat  similar  manner,  over  fused  tin,  with 
but  partial  success ;  he  also  made  many  other  attempts  to 
obtain  a  perfect  vacuum ;  his  main  object  being  to  ascertain 
what  would  be  the  effect  of  electricity  across  empty  space  : 


LIGHT.  135 

he  admits  that  he  could  not  succeed  in  procuring  a  vacuum, 
but  found  electricity  much  less  readily  conducted  or  trans- 
mitted by  the  best  vacuum  he  could  procure  than  by  the  ordi- 
nary Boylean  vacuum. 

Morgan  found  no  conduction  by  a  good  Torricellian  vac- 
uum ;  and,  although  Davy  does  not  seem  to  place  much  reliance 
on  Morgan's  experiments,  there  was  one  point  in  which  they 
were  less  liable  to  error  than  those  of  Davy.  Morgan,  whose 
experiments  seem  to  have  been  carefully  conducted,  operated 
with  hermetically-sealed  glass  tubes  and  by  induced  electricity, 
while  Davy  sealed  a  platinum  wire  into  the  extremity  of  the 
tube  in  which  he  sought  to  produce  a  vacuum.  I  have  found 
in  very  numerous  experiments  which  I  made  to  exclude  air 
from  water,  that  platinum  wires,  most  carefully  sealed  into 
glass,  allow  liquids  to  pass  between  them  and  the  glass  ;  and 
this  gives  every  reason  to  believe  that  gases  may  equally  pass 
through  ;  I  have  observed  such  effect  in  the  gas  battery  when 
it  has  been  in  action  for  a  long  period.  Davy  supposed  that 
the  particles  of  bodies  may  be  detached,  and  so  produce  elec- 
trical effects  in  a  vacumm ;  and  such  effects  would  more  read- 
ily take  place  in  his  experiments,  where  a  wire  projected 
into  the  exhausted  space,  than  in  Morgan's,  where  the  in- 
duced electricity  was  diffused  over  the  surface  of  the  glass. 

M.  Masson  found  that  the  barometric  vacuum  does  not 
conduct  a  current  of  electricity,  or  even  a  discharge,  unless 
the  tension  is  considerable  and  sufficient  to  detach  particles 
from  the  electrodes ;  and  by  adopting  a  plan  of  Dr.  An- 
drews, viz.  absorbing  carbonic  acid  by  potash,  M.  Gassiot 
has  recently  succeeded  in  forming  vacua  across  which 
the  powerful  discharge  from  the  Bhumkorf  coil  will  not 
pass. 

The  odour  which  many  metals,  such  as  iron,  tin,  and 
zinc  emit,  and  the  so-called  thermographic  radiations,  we 
can  hardly  explain  upon  any  other  theory  than  the  evapora- 
tion of  an  innnitesimally  small  portion  of  the  metal  itself.  •* 


136  CORRELATION   OF  PHYSICAL   FORCES. 

So  universal  is  the  tendency  of  matter  to  diffuse  itself 
into  space ,  that  it  gave  rise  to  the  old  saying  that  nature 
abhors  a  vacuum ;  an  aphorism  which,  though  cavilled  at  and 
ridiculed  by  the  self-sufficiency  of  some  modern  philosophers, 
contains  in  a  terse,  though  somewhat  metaphorical,  form  of 
expression  a  comprehensive  truth,  and  evinces  a  large  extent 
of  observation  in  those  who,  with  few  of  the  advantages  which 
we  possess,  first  generalised  by  this  sentence  the  facts  of 
which  they  had  become  cognisant. 

It  has  been  argued  that,  if  matter  were  capable  of  infinite 
divisibility,  the  earth's  atmosphere  would  have  no  limit,  and 
that  consequently  portions  of  it  would  exist  at  points  of  space 
where  the  attraction  of  the  sun  and  planets  would  be  greater 
than  that  of  the  earth,  and  whence  it  would  fly  off  to  those 
bodies  and  form  atmospheres  around  them.  This  was  sup- 
posed to  be  negatived  by  the  argument  of  the  well-known 
paper  of  Dr.  Wollaston  ;  in  which,  from  the  absence  of  nota- 
ble refraction  near  the  margin  of  the  sun  and  of  the  planet 
Jupiter,  he  considered  himself  entitled  to  conclude  that  the 
expansion  of  the  earth's  atmosphere  had  a  definite  limit,  and 
was  balanced  at  a  certain  point  by  gravitation :  this  deduc- 
tion has  been  shown  to  be  inconclusive  by  Dr.  Whewell,  and 
has  also  been  impugned  upon  others  grounds  by  Dr.  Wilson. 
There  is  a  point  not  adverted  to  in  these  papers,  and  which 
Wollaston  does  not  seem  to  have  considered,  viz.  that  there 
is  no  evidence  that  the  apparent  discs  of  the  sun  and  of  Jupi- 
ter show  us  their  real  discs  or  bodies.  Sir.  W.  Herschel 
regards  the  margin  of  the  visible  discs  as  that  of  clouds  or  a 
peculiar  state  of  atmosphere,  and  the  rapidly  changing  char- 
acter of  the  apparent  surfaces  render  some  such  conclusion 
necessary.  If  this  be  so,  refraction  of  an  occulted  star  could* 
not  be  detected — at  all  events,  in  the  denser  portion  of  the 
atmosphere. 

Sir  W.  Herschel's  observations  go  to  prove  that  the 
eun  and  Jupiter  have  dense  atmospheres,  while  Wollaston's 


LIGHT.  137 

were  believed  to  prove  that  they  have  no  appreciable  atmos- 
pheres. 

If  it  be  admitted,  or  considered  proved,  that  the  sun  and 
planets  have  atmospheres—- and  little  doubt  now  exists  on  this 
point — then  the  grounds  upon  which  Wollaston  founded  his 
arguments  are  untenable  ;  and  there  appears  no  reason  why 
the  atmosphere  of  the  different  planets  should  not  be,  with 
reference  to  each  other,  in  a  state  of  equilibrium.  Ether,  or 
the  highly-attenuated  matter  existing  in  the  interplanetary 
spaces,  being  an  expansion  of  some  or  all  of  these  atmos- 
pheres, or  of  the  more  volatile  portions  of  them,  would  thus 
furnish  matter  for  the  transmission  of  the  modes  of  motion 
which  we  call  light,  heat,  &c. ;  and  possibly  minute  portions 
of  these  atmospheres  may,  by  gradual  changes,  pass  from 
planet  to  planet,  forming  a  link  of  material  communication 
between  the  distant  monads  of  the  universe. 

The  view  given  above  would  approximate  the  theory  of 
the  transmission  of  light  by  the  undulations  of  ordinary  mat- 
ter to  the  other  two  theories,  which  equally  suppose  the  non- 
existence  of  a  vacuum ;  for,  according  to  the  emissive  or 
corpuscular  theory,  the  vacuum  is  filled  by  the  matter  itself, 
of  light,  heat,  &c. ;  according  to  the  ethereal,  it  is  filled  by 
the  all-penetrating  ether.  Of  the  existence  of  matter  in  the 
interplanetary  spaces  we  have  some  evidence  in  the  diminish- 
ing periods  of  comets  ;  and  where,  from  its  highly  attenuated 
state,  the  character  of  the  medium  by  which  the  forces  are 
conveyed  cannot  be  tested,  the  term  ether  is  a  most  appropri- 
ate generic  name  for  such  medium. 

Newton  has  some  curious  passages  on  the  subject  matter 
of  light.  In  the  '  Queries  to  the  Optics '  he  says  : — 
^  +  Are  not  gross  bodies  and  light  convertible  into  one 
another,  and  may  not  bodies  receive  much  of  their  activity 
from  the  particles  of  light  which  enter  their  composition? 
*  *  *  The  changing  of  bodies  into  light  and  light  into 
bodies  is  very  conformable  to  the  course  of  nature,  which 


138  CORRELATION    OF   PHYSICAL    FORCES. 

seems  delighted  with  transmutations.  Water,  which  is  a 
very  fluid,  tasteless  salt,  she  changes  by  heat  into  vapour, 
which  is  a  sort  of  air,  and  by  cold  into  ice,  which  is  a  hard, 
pellucid,  brittle,  fusible  stone,  and  this  stone  returns  into 
water  by  heat,  and  vapour  returns  into  water  by  cold.  *  * 
And,  among  such  various  and  strange  transmutations,  why 
may  not  nature  change  bodies  into  light,  and  light  into 
bodies  ? ' 

Newton  has  here  seemingly  in  his  mind  the  emissive 
theory  of  light ;  but  the  passages  might  be  applied  to  either 
theory ;  the  analogy  he  saw  in  the  change  of  state  of  matter, 
as  in  ice,  water,  and  vapour,  with  the  hypothetic  change  into 
light,  is  very  striking,  and  would  seem  to  show  that  he  regard- 
ed the  change  or  transmutation  of  which  he  speaks  as  one 
analogous  to  the  known  changes  of  state,  or  consistence,  in 
ordinary  matter. 

The  difference  between  the  view  which  1  am  advocating 
and  that  of  the  ethereal  theory  as  generally  enunciated  is, 
that  the  matter  which  in  the  interplanetary  spaces  serves  as 
the  means  of  transmitting  by  its  undulations  light  and  heat,  I 
should  regard  as  possessing  the  qualities  of  ordinary,  or  as  it 
has  sometimes  been  called  gross,  matter,  and  particularly 
weight ;  though,  from  its  extreme  rarefaction,  it  would  mani- 
fest these  properties  in  an  indefinitely  small  degree  ;  whilst,  on 
the  surface  of  the  earth,  that  matter  attains  a  density  cognisa- 
ble by  our  means  of  experiment,  and  the  dense  matter  is 
itself,  in  great  part,  the  conveyer  of  the  undulations  in  which 
these  agents  consist.  Doubtless,  in  very  many  of  the  forms 
which  matter  assumes  it  is  porous,  and  pervaded  by  more 
volatile  essences,  which  may  differ  as  much  in  kind  as  matter 
does.  In  these  cases  a  composite  medium,  such  a«  that  indi- 
cated by  Dr.  Young,  would  result ;  but  even  on  such  a  suppo- 
sition, the  denser  matter  would  probably  exercise  the  more 
important  influence  on  the  undulations.  Returning  to  the 
somewhat  strained  hypothesis,  that  the  particles  of  dense 


LIGHT.  139 

matter  in  a  so-called  solid  are  as  distant  as  the  stars  in  heaven, 
still  a  certain  depth  or  thickness  of  such  solid  would  present 
at  every  point  of  space  a  particle  or  rock  in  the  successive 
progress  of  a  wave,  which  particles,  to  carry  on  the  move- 
ment, must  vibrate  in  unison  with  it.  „ 

At  the  utmost,  our  assumption,  on  the  one  hand,  is  that 
wherever  light,  heat,  &c.,  exist,  ordinary  matter  exists,  though 
it  may  be  so  attenuated  that  we  cannot  recognise  it  by  the 
tests  of  other  forces,  such  as  gravitation,  and  that  to  the  ex- 
pansibility of  matter  no  limit  can  be  assigned.  On  the  other 
hand,  a  specific  matter  without  weight  must  be  assumed,  of 
the  existence  of  which  there  is  no  evidence,  but  in  the  phe- 
nomena for  the  explanation  of  which  its  existence  is  supposed. 
To  account  for  the  phenomena  the  ether  is  assumed,  and  to 
prove  the  existence  of  the  ether  the  phenomena  are  cited. 
For  these  reasons,  and  others  above  given,  I  think  that  the 
assumption  of  the  universality  of  ordinary  matter  is  the  least 
gratuitous. 

OuSep  TI  rov  iravros  KSVOV  TreAet  ovSe  irspiffffov. 

A  question  has  often  occurred  to  me  and  possibly  to  oth- 
ers :  Is  the  continuance  of  a  luminous  impulse  in  the  inter- 
planetary spaces  perpetual,  or  does  it  after  a  certain  distance 
dissipate  itself  and  become  lost  as  light — I  do  not  mean  by 
mere  divergence  directly  as  the  squares  of  the  distances  it 
travels,  but  does  the  physical  impulse  itself  lose  force  as  it 
proceeds?  Upon  the  view  I  have  advocated,  and  indeed 
upon  any  undulatory  hypothesis,  there  must  be  some  resist- 
ance to  its  progress  ;  and  unless  the  matter  or  ether  in  the 
interplanetary  spaces  be  infinitely  elastic,  .and  there  be  no 
lateral  action  of  a  ray  of  light,  there  must  be  some  loss. 
That  it  is  exceedingly  minute  is  proved  by  the  distance  light 
travels.  Stars  whose  parallax  is  ascertained  are  at  such  a 
distance  from  the  earth  that  their  light,  travelling  at  the  rate 
of  192,500  miles  in  a  second,  takes  more  than  ten  years  to 


140  CORRELATION  OF  PHYSICAL   FORCES. 

reach  the  earth ;  so  that  we  see  them  as  they  existed  ten 
years  ago.  The  distance  of  most  visible  stars  is  probably  far 
greater  than  this,  and  yet  their  brilliance  is  great,  and  in- 
creases when  their  rays  are  collected  by  the  telescope  in  pro- 
portion ceteris  paribus  to  the  area  of  the  object-glass  or  spec- 
ulum. There  is,  however,  an  argument  of  a  somewhat  spec- 
ulative character,  by  which  light  would  seem  to  be  lost  or 
transformed  into  some  other  force  in  the  interplanetary  spaces. 

Every  increase  of  space-penetrating  power  in  the  tele- 
scope gives  us  a  new  field  of  visible  stars.  If  this  expansion 
of  the  stellar  universe  go  on  indefinitely  and  no  light  be  lost, 
then,  assuming  the  fixed  stars  to  be  of  an  average  equal 
brightness  with  our  sun,  and  no  light  lost  other  than  by  diver- 
gence, the  night  ought  to  be  equally  luminous  with  the  day  ; 
for  though  the  light  from  each  point  diminishes  in  intensity 
as  the  square  of  the  distance,  the  number  of  luminous  points 
would  fill  up  the  whole  space  around  us  ;  and  if  every  point 
of  space  is  occupied  by  an  equally  brilliant  point  of  light,  the 
distance  of  the  points  becomes  immaterial.  The  loss  of  light 
intercepted  by  stellar  bodies  would  make  no  difference  in  the 
total  quantity  of  light,  for  each  of  these  would  yield  from  its 
own  self-luminosity  at  least  as  much  light  as  it  intercepted. 
Light  may,  however,  be  intercepted  by  opaque  bodies,  such 
as  planets  ;  but,  making  every  allowance  for  these,  it  is  diffi- 
cult to  understand  why  we  get  so  little  light  at  night  from  the 
stellar  universe,  without  assuming  that  some  light  is  lost  in  its 
progress  through  space — not  lost  absolutely,  for  that  would  be 
an  annihilation  of  force — but  converted  into  some  other  mode 
of  motion. 

It  may  be  objected  that  this  hypothesis  assumes  the  stel- 
lar universe  to  be  illimitable  :  if  pushed  to  its  extreme  so  as 
to  make  the  light  of  night  equal  that  of  day,  provided  no 
stellar  light  be  lost,  it  does  make  this  assumption  ;  but  even 
this  is  a  far  more  rational  assumption  to  make  than  that  the 
stellar  universe  is  limited.  Our  experience  gives  no  indica- 


LIGHT.  14:1 

tion  of  a  limit ;  each  improvement  in  telescopic  power  gives 
us  new  realms  of  stars  or  of  nebulas,  which,  if  not  stellar 
clusters,  are  at  all  events  self-luminous  matter  ;  and  if  we  as- 
sume a  limit,  what  is  it?  We  cannot  conceive  a  physical 
boundary,  for  then  immediately  comes  the  question,  what 
bounds  the  boundary  ?  and  to  suppose  the  stellar  universe  to 
be  bounded  by  infinite  space  or  by  infinite  chaos,  that  is  to 
say,  to  suppose  a  spot — for  it  would  then  become  so— of  mat- 
ter in  definite  forms,  with  definite  forces,  and  probably  teem- 
ing with  definite  organic  beings,  plunged  in  a  universe  of 
nothing,  is  to  my  mind  at  least  far  more  unphilosophical  than 
to  suppose  a  boundless  universe  of  matter  existing  in  forms 
and  actions  analogous  to  those  which,  as  far  as  our  examina- 
tion goes,  pervade  space.  But  without  speculating  on  topics 
in  which  the  mind  loses  itself,  it  may  not  unreasonably  be 
expected  that  a  greater  amount  of  light  would  reach  us  from 
the  surrounding  self-luminous  spheres  were  not  some  portion 
lost  as  light,  by  its  action  on  the  medium  which  conveys  the 
impulses.  What  force  this  becomes,  or  what  it  effects,  it 
would  be  idle  to  speculate  upon. 


VI.— MAGNETISM. 

MAGNETISM,  as  was  proved  by  the  important  discov- 
ery of  Faraday,  will  produce  electricity,  but  with  this 
peculiarity — that  in  itself  it  is  static  ;  and,  therefore,  to  pro- 
duce a  dynamic  force,  motion  must  be  superadded  to  it :  it  is, 
in  fact,  directive,  not  motive,  altering  the  direction  of  other 
forces,  but  not,  in  strictness,  initiating  them. .  It  is  difficult 
to  convey  a  definite  notion  of  the  force  of  magnetism,  and  of 
the  mode  in  which  it  affects  other  forces.  The  following  il- 
lustration may  give  a  rude  idea  of  magnetic  polarity.  Sup- 
pose a  number  of  wind-vanes,  say  of  the  shape  of  arrows, 
with  the  spindles  on  which  they  revolve  arranged  in  a  row, 
but  the  vanes  pointing  in  various  directions  :  a  wind  blowing 
from  the  same  point  with  an  uniform  velocity  will  instantly 
arrange  these  vanes  in  a  definite  direction,  the  arrow-heads 
or  narrow  parts  pointing  one  way,  the  swallow-tails  or  broad 
parts  another.  If  they  be  delicately  suspended  on  their  spin- 
dles, a  very  gentle  breeze  will  so  arrange  them,  and  a  very 
gentle  breeze  will  again  deflect  them ;  or,  if  the  wind  cease, 
and  they  have  been  originally  subject  to  other  forces,  such  as 
gravity  from  unequal  suspension,  they  will  return  to  irregu- 
lar positions,  themselves  creating  a  slight  breeze  by  their  re- 
turn. Such  a  state  of  things  will  represent  the  state  of  the 
molecules  of  soft  iron  ;  electricity  acting  on  them — not  indeed 
in  straight  lines,  but  in  a  definite  direction — produces  a  polar 


MAGNETISM.  143 

arrangement,  which  they  will  lose  as  soon  as  the  dynamic  in- 
ducing force  is  removed. 

Let  us  now  suppose  the  vanes,  instead  of  turning  easily, 
to  be  more  stiffly  fixed  to  the  axles,  so  as  to  be  turned  with 
difficulty :  it  will  require  a  stronger  wind  to  move  them  and 
arrange  them  definitely ;  but  when  so  arranged,  they  will  re- 
tain their  position  ;  and  should  a  gentle  breeze  spring  up  in 
another  direction,  it  will  not  alter  their  position,  but  will  it- 
self be  definitely  deflected.  Should  the  conditions  of  force 
and  stability  be  intermediate,  both  the  breeze  and  the  vanes 
will  be  slightly  deflected ;  or,  if  there  be  no  breeze,  and  the 
spindles  be  all  moved  in  any  direction,  preserving  their  linear 
relation,  they  will  themselves  create  a  breeze.  Thus  it  is 
with  the  molecules  of  hard  iron  or  steel  in  permanent  mag- 
nets ;  they  are  polarised  with  greater  difficulty,  but,  when  so 
polarised,  they  cannot  be  affected  by  a  feeble  current  of  elec- 
tricity. Again,  if  the  magnets  be  moved,  they  themselves 
originate  a  current  of  electricity ;  and,  lastly,  the  magnetic 
polarity  and  the  electric  current  may  be  both  mutually  af- 
fected, if  the  degrees  of  motion  and  stability  be  intermediate. 

The  above  instance  will,  of  course,  be  taken  only  as  an 
approximation,  and  not  as  binding  me  to  any  closer  analogy 
than  is  generally  expected  of  a  mechanical  illustration.  It 
is  difficult  to  convey  by  words  a  definite  idea  of  the  dual  or 
antithetic  character  of  force  involved  in  the  term  polarity. 
The  illustration  I  have  employed"  may,  I  hope,  somewhat  aid 
in  elucidating  the  manner  in  which  magnetism  acts  on  the 
other  dynamic  forces  ;  i.  e.,  definitely  directing  them,  but  not 
initiating  them,  except  while  in  motion. 

Magnets  being  moved  in  the  direction  of  lines,  joining 
their  poles,  produce  electrical  currents  in  such  neighbouring 
bodies  as  are  conductors  of  electricity,  in  directions  trans- 
verse to  the  line  of  motion ;  and  if  the  direction  of  motion 
or  the  position  of  the  magnetic  poles  be  reversed,  the  current 
of  electricity  flows  in  a  reverse  direction.  So  if  the  magnet 


144  CORRELATION   OF  PHYSICAL  FORCES. 

be  stationary,  conducting  bodies  moved  across  any  of  the 
lines  of  magnetic  force,  i.  e.  lines  in  the  direction  of  which 
the  mutual  action  of  the  poles  of  the  magnet  would  place 
minute  portions  of  iron,  have  currents  of  electricity  devel- 
oped in  them,  the  direction  of  which  is  dependent  upon  that 
of  the  motion  of  the  substance  with  reference  to  the  magnetic 
poles.  Thus,  as  bodies  affected  by  an  electrical  current  are 
definitely  moved  by  a  magnet  in  proximity  to  them,  so  con- 
versely bodies  moved  near  a  magnet  have  an  electrical  cur- 
rent developed  in  them.  Magnetism  can,  then,  through  the 
medium  of  electricity,  produce  heat,  light,  and  chemical  affin- 
ity. Motion  it  can  directly  produce  under  the  above  condi- 
tions ;  i.  e.  a  magnet  being  itself  moved  will  move  other  fer- 
reous  bodies  :  these  will  acquire  a  static  condition  of  equilib- 
rium, and  be  again  moved  when  the  magnet  is  also  moved. 
By  motion  or  arrested  motion  only,  could  the  phenomena  of 
magnetism  ever  have  become  known  to  us.  A  magnet,  how- 
ever powerful,  might  rest  for  ever  unnoticed  and  unknown, 
unless  it  were  moved  near  to  iron,  or  iron  moved  near  to  it, 
so  as  to  come  within  the  sphere  of  its  attraction. 

But  even  with  other  than  either  magnetic  or  electrified 
substances,  all  bodies  will  be  moved  when  placed  near  the 
poles  of  very  powerful  magnets — some  taking  a  position  ax- 
ially,  or  in  the  line  from  pole  to  pole  of  the  magnet ;  others 
equatorially,  or  in  a  direction  transverse  to  that  line — the 
former  being  attracted,  the  latter  apparently  repelled,  by  the 
poles  of  the  magnet.  These  effects,  according  to  the  views 
of  Faraday,  show  a  generic  difference  between  the  two 
classes  of  bodies,  magnetics  and  diamagnetics ;  according  to 
others,  a  difference  of  degree  or  a  resultant  of  magnetic  ac- 
tion ;  the  less  magnetic  substance  being  forced  into  a  trans- 
verse position  by  the  magnetisation  of  the  more  magnetic 
medium  which  surrounds  it. 

According  to  the  view  given  above,  magnetism  may  be 
produced  by  the  other  forces,  just  as  the  vanes  in  the  instance 


MAGNETISM.  145 

given  are  definitely  deflected,  but  cannot  produce  them  except 
when  in  motion :  motion,  therefore,  is  to  be  regarded  in  this 
case  as  the  initiative  force.  Magnetism  will,  however,  di- 
rectly affect  the  other  forces — light,  heat,  and  chemical  affin- 
ity, and  change  their  direction  or  mode  of  action,  or,  at  all 
events,  will  so  affect  matter  subjected  to  these  forces,  that 
their  direction  is  changed.  Since  these  lectures  were  deliv- 
ered, Faraday  has  discovered  a  remarkable  effect  of  the  mag- 
netic force  in  occasioning  the  deflection  of  a  ray  of  polarised 
light. 

If  a  ray  of  polarised  light  pass  through  water,  or  through 
any  transparent  liquid  or  solid  which  does  not  alter  or  turn 
aside  the  plane  of  polarisation,  and  the  column,  say  of  water, 
through  which  it  passes  be  subjected  to  the  action  of  a  pow- 
erful magnet,  the  line  of  magnetic  force,  or  that  which  would 
unite  the  poles  of  the  magnet,  being  in  the  same  direction  as 
the  ray  of  polarised  light,  the  water  acquires,  with  reference 
to  the  light,  similar,  though  not  quite  identical,  properties  to 
oil  of  turpentine — the  plane  of  polarisation  is  rotated,  and 
the  direction  of  this  rotation  is  changed  by  changing  the  di- 
rection of  the  magnetic  force :  thus,  if  we  suppose  a  polar- 
ised ray  to  pass  first  in  its  course  the  north  pole  of  the  mag- 
net, then  between  that  and  the  south  pole  it  will  be  deflected, 
or  curved  to  the  right ;  while  if  it  meets  the  south  pole  first 
in  its  course,  it  will,  in  its  journey  between  that  and  the  north 
pole,  be  turned  to  the  left.  If  the  substance  through  which 
the  ray  is  transmitted  be  of  itself  capable  of  deflecting  the 
plane  of  polarisation,  as,  for  instance,  oil  of  turpentine,  then 
the  magnetic  influence  will  increase  or  diminish  this  rotation, 
according  to  its  direction.  A  similar  effect  to  this  is  observed 
with  polarised  heat  when  the  medium  through  which  it  is 
transmitted  is  subjected  to  magnetic  influence. 

Whether  this  effect  of  magnetism  is  rightly  termed  an  ef- 
fect upon  light  and  heat,  or  is  a  molecular  change  of  the  mat- 
ter transmitting  the  light  and  heat,  is  a  question  the  resolu- 
7 


146  CORRELATION  OF  PHYSICAL  FORCES. 

tion  of  which  must  be  left  to  the  future  ;  at  present,  the  an- 
swer to  it  would  depend  upon  the  theory  we  adopt.  If  the 
view  of  light  and  heat  which  I  have  stated  be  adopted,  then 
we  may  fairly  say  that  magnetism,  in  these  experiments,  di- 
rectly affects  the  other  forces ;  for  light  and  heat  being,  ac- 
cording to  that  view,  motions  of  ordinary  matter,  magnetism, 
in  affecting  these  movements,  affects  the  forces  which  occa- 
sion them.  If,  however,  the  other  theories  be  adhered  to,  it 
would  be  more  consistent  with  the  facts  to  view  these  results 
as  exhibiting  an  action  upon  the  matter  itself,  and  the  heat 
and  light  as  secondarily  affected. 

When  substances  are  undergoing  chemical  changes,  and  a 
magnet  is  brought  near  them,  the  direction  or  lines  of  action 
of  the  chemical  force  will  be  changed.  There  are  many  old 
experiments  which  probably  depended  on  this  effect,  but 
which  were  erroneously  considered  to  prove  that  permanent 
magnetism  could  produce  or  increase  chemical  action :  these 
have  recently  been  extended  and  explained  by  Mr.  Hunt  and 
Mr.  Wartmann,  and  are  now  better  understood. 

The  above  cases  are  applicable  to  the  subject  of  the  pres- 
ent Essay,  inasmuch  as  they  show  a  relation  to  exist  between 
magnetic  and  the  other  forces,  which  relation  is,  in  all  proba- 
bility, reciprocal ;  but  in  these  cases  there  is  not  a  production 
of  light,  heat,  or  chemical  affinity,  by  magnetism,  but  a  change 
in  their  direction  or  mode  of  action. 

There  is,  however,  that  which  may  be  viewed  as  a  dy- 
namic condition  of  magnetism  ;  i.  e.  its  condition  at  the  com- 
mencement and  the  termination,  or  during  the  increment  or 
decrement  of  its  development.  While  iron  or  steel  is  being 
rendered  magnetic,  and  as  it  progresses  from  its  non-magnetic 
to  its  maximum  magnetic  state,  or  recedes  from  its  maximum 
to  zero,  it  exhibits  a  dynamic  force  ;  the  molecules  are,  it 
may  be  inferred,  in  motion.  Similar  effects  can  then  be  pro- 
duced to  those  which  are  produced  by  a  magnet  whilst  in  mo- 
tion. 


MAGNETISM.  147 

An  experiment  which  I  published  in  1845  tends,  I  think, 
to  illustrate  this,  and  in  some  degree  to  show  the  character 
of  the  motion  impressed  upon  the  molecules  of  a  magnetic 
metal  at  the  period  of  magnetisation.  A  tube  filled  with  the 
liquid  in  which  magnetic  oxide  of  iron  had  been  prepared, 
and  terminated  at  each  end  by  plates  of  glass,  is  surrounded 
by  a  coil  of  coated  wire.  To  a  spectator  looking  through  this 
tube  a  flash  of  light  is  perceptible  whenever  the  coil  is  elec- 
trised, and  less  light  is  transmitted  when  the  electrical  current 
ceases,  showing  a  symmetrical  arrangement  of  the  minute 
particles  of  magnetic  oxide  while  under  the  magnetic  in- 
fluence. 

In  this  experiment  it  should  be  borne  in  mind,  that  the 
particles  of  oxide  of  iron  are  not  shaped  by  the  hand  of  man, 
as  would  be  the  case  with  iron  filings,  or  similar  minute  por- 
tions of  magnetic  matter,  but  being  chemically  precipitated, 
are  of  the  form  given  to  them  by  nature. 

While  magnetism  is  in  the  state  of  change  above  described, 
it  will  produce  the  other  forces ;  but  it  may  be  said,  while 
magnetism  is  thus  progressive,  some  other  force  is  acting  on 
it,  and  therefore  it  does  not  initiate  :  this  is  true,  but  the 
same  may  be  said  of  all  the  other  forces  ;  they  have  no  com- 
mencement that  we  can  trace.  We  must  ever  refer  them 
back  to  some  antecedent  force  equal  in  amount  to  that  pro- 
duced, and  therefore  the  word  initiation  cannot  in  strictness 
apply,  but  must  only  be  taken  as  signifying  the  force  selected 
as  the  first :  this  is  another  reason  why  the  idea  of  abstract 
causation  is  inapplicable  to  physical  production.  To  this 
point  I  shall  again  advert. 

Electricity  may  thus  be  produced  directly  by  magnetism, 
either  when  the  magnet  as  a  mass  is  in  motion,  or  when  its 
magnetism  is  commencing,  increasing,  decreasing,  or  ceasing ; 
and  heat  may  similarly  be  directly  produced  by  magnetism. 
I  have,  since  the  first  edition  of  this  Essay  was  published, 
communicated  to  the  Royal  Society  a  paper  by  which  I  think 


14:8  CORRELATION   OF  PHYSICAL   FORCES. 

[  have  satisfactorily  proved,  that  whenever  any  metal  suscepti- 
ble of  magnetism  is  magnetised  or  demagnetised,  its  tempera- 
ture is  raised.  This  was  shown,  first,  by  subjecting  a  bar  of 
iron,  nickel,  or  cobalt  to  the  influence  of  a  powerful  electro- 
magnet, which  was  rapidly  magnetised  and  demagnetised  in 
reverse  directions,  the  electro-magnet  itself  being  kept-  cool  by 
cisterns  of  water,  so  that  the  magnetic  metal  subjected  to  the 
influence  of  magnetism  was  raised  to  a  higher  temperature 
than  the  electro-magnet  itself,  and  could  not,  therefore,  have 
acquired  its  increased  temperature  by  conduction  or  radiation 
rf  heat  from  the  electro- magnet ;  and  secondly,  by  rotating 
a,  permanent  steel  magnet  with  its  pole  opposite  to  a  bar 
;>f  iron,  a  thermo-electric  pile  being  placed  opposite  the 
latter. 

Dr.  Maggi  covered  a  plate  of  homogeneous  soft  iron  with 
a,'  thin  coating  of  wax  mixed  with  oil,  a  tube  traversed  the 
centre  through  which  the  vapour  of  boiling  water  was  passed, 
rhe  plate  was  made  to  rest  on  the  poles  of  an  electro-magnet, 
with  card  interposed.  When  the  iron  is  not  magnetised,  the 
melted  wax  assumes  a  circular  form,  the  tube  occupying  the 
centre,  but  when  the  electro-magnet  is  put  in  action,  the  curve 
marking  the  boundary  of  the  melted  substance  changes  its  form 
md  becomes  elongated  in  a  direction  transverse  to  the  line 
ioining  the  poles,  showing  that  the  conducting  power  of  the 
iron  for  heat  is  changed  by  magnetisation. 

Thus  we  get  heat  produced  by  magnetism  and  the  conduc- 
tion of  heat  altered  by  it  in  a  direction  having  a  definite  rela- 
ion  to  the  direction  of  the  magnetism.  Is  it  necessary  to 
2all  in  aid  ethep  or  the  substance  c  caloric*  to  explain  these 
results  ?  is  it  not  more  rational  to  regard  the  calorific  effects 
is  changes  in  the  molecular  arrangements  of  the  matter  sub- 
jected to  magnetism  ? 

There  is  every  probability  that  magnetism,  in  the  dyna- 
mic state,  either  when  the  magnet  is  in  motion,  or  when  the 
magnetic  intensity  is  varying,  will  also  directly  produce  chemi- 


MAGNETISM.  1 

cal  affinity  and  light,  though,  up  to  the  present  time,  such  I 
not  been  proved  to  be  the  case  ;  the  reciprocal  effect,  also, 
the  direct  production  of  magnetism  by  light  and  heat  has  r 
yet  been  experimentally  established. 

I  have  used,  in  contradistinction,  the  terms  dynamic  a 
static  to  represent  the  different  states  of  magnetism.  1 
applications  I  have  made  of  these  terms  may  be  open  to  so 
exception,  but  I  know  of  no  other  words  which  will  so  nea 
express  my  meaning. 

The  static  condition  of  magnetism  resembles  the  sta 
condition  of  other  forces :  such  as  the  state  of  tension  exi 
ing  in  the  beam  and  a  cord  of  a  balance,  or  in  a  charg 
Leyden  phial.  The  old  definition  of  force  was,  that  wh: 
caused  change  in  motion ;  and  yet  even  this  definition  p 
sents  a  difficulty :  in  a  case  of  static  equilibrium,  such, 
instance,  as  that  which  obtains  in  the  two  arms  of  a  balan 
we  get  the  idea  of  force  without  any  palpable  apparent  motio 
whether  there  be  really  an  absence  of  motion  may  be  a  dou 
ful  question,  as  such  absence  would  involve  in  this  case  p 
feet  elasticity,  and,  in  all  other  cases,  a  stability  which,  ii 
long  course  of  time,  nature  generally  negatives,  showing, 
I  believe,  an  inseparable  connection  of  motion  with  matt 
and  an  impossibility  of  a  perfectly  immobile  or  durable  sta 
So  with  magnetism:  I  believe  no  magnet  can  exist  in 
absolutely  stable  state,  though  the  duration  of  its  stabil 
will  be  proportionate  to  its  original  resistance  to  assumi 
a  polarised  condition.  This,  however,  must  be  taken  mer< 
as  a  matter  of  opinion  :  we  have,  in  support  of  it,  the  gene] 
facts  that  magnets  do  deteriorate  in  the  course  of  years  ;  a 
we  have  the  further  general  fact  of  the  instability,  or  fluxioi 
state,  of  all  nature,  when  we  have  an  opportunity  of  fai] 
investigating  it  at  different  and  remote  periods :  in  ma 
cases,  however,  the  action  is  so  slow  that  the  changes  esca 
human  observation,  and,  until  this  can  be  brought  to  be 
over  a  proportionate  period  of  time,  the  proposition  cannot 


150  COEEELATION   OF  PHYSICAL   FOKCES. 

said  to  be  experimentally  or  inductively  proved,  but  must  be 
left  to  the  mental  conviction  of  those  who  examine  it  by  the 
light  of  already  acknowledged  facts. 

All  cases  of  static  force  present  the  same  difficulty :  thus, 
two  springs  pressing  against  each  other  would  be  said  to  be 
exercising  force  ;  and  yet  there  is  no  resulting  action,  no  heat, 
no  light,  &c. 

So  if  gas  be  compressed  by  a  piston,  at  the  time  of  com- 
pression heat  is  given  off;  but  when  this  is  abstracted, 
although  the  pressure  continues,  no  further  heat  is  eliminated. 
Thus,  by  an  equilibrium  produced  by  opposing  forces,  motion 
is  locked  up,  or  in  abeyance,  as  it  were,  and  may  be  again 
developed  when  the  forces  are  relieved  from  the  tension. 
But  in  the  first  instance,  in  producing  the  state  of  tension, 
force  has  to  be  employed ;  and  as  we  have  said  in  treating  of 
mechanical  force,  so  with  the  other  forces  the  original  change 
which  disturbs  equilibrium  produces  other  changes  which  go 
on  without  end.  Thus,  by  the  act  of  charging  a  Leyden 
phial,  the  cylinder,  the  rubber,  and  the  adjoining  portions  of 
the  electrical  machine  have  each  and  all  their  states  changed, 
and  thence  produce  changes  in  surrounding  bodies  ad  infini- 
tum ;  when  the  jar  is  discharged,  converse  changes  are  again 
produced. 

As  with  heat,  light,  and  electricity,  the  daily  accumulating 
observations  tend  to  show  that  each  change  in  the  phenomena 
to  which  these  names  are  given  is  accompanied  by  a  change 
either  temporary  or  permanent  in  the  matter  affected  by  them ; 
so  many  recent  experiments  on  magnetism  have  connected 
magnetic  phenomena  with  a  molecular  change  in  the  subject 
matter.  Thus  M.  Wertheim  has  shown  that  the  elasticity  of 
iron  and  steel  is  altered  by  magnetisation  ;  the  co-efficient  of 
elasticity  in  iron  being  temporarily,  in  steel  permanently 
diminished. 

He  has  also  examined  the  effects  of  torsion  upon  magnet- 
ised iron,  and  concludes,  from  his  experiments,  that  in  a  bar 


MAGNETISM.  151 

of  iron  arrived  at  a  state  of  magnetic  equilibrium,  temporary 
torsion  diminishes  the  magnetism,  and  that  the  untwisting  or 
return  to  its  primitive,  state  restores  the  original  degree  of 
magnetisation. 

M.  Gruillemin  observed  that  a  bar  slightly  curved  by  its 
own  weight  is  straightened  by  being  magnetised.  Mr.  Page 
and  Mr.  Marrion  discovered  that  a  sound  is  emitted  when 
iron  or  steel  is  rapidly  magnetised  or  demagnetised ;  and  Mr. 
Joule  found  that  a  bar  of  iron  is  slightly  elongated  by  mag- 
netisation. 

Again,  with  regard  to  diamagnetic  bodies,  M.  Matteucci 
found  that  the  mechanical  compression  of  glass  altered  the 
rotatory  power  upon  a  ray  of  polarised  light  which  it  trans- 
mitted. He  further  considered  that  a  change  took  place  in 
the  temper  of  portions  of  glass  which  he  submitted  to  the  in- 
fluence of  powerful  magnets. 

The  same  arguments  which  have  been  submitted  to 
the  reader  as  to  the  other  affections  of  matter  being  modes 
of  molecular  motion,  are  therefore  equally  applicable  to  mag- 
netism. 


YIL— CHEMICAL  AFFINITY. 

/CHEMICAL  AFFINITY,  or  the  force  by  which  dissimi- 
\_J  lar  bodies  tend  to  unite  and  form  compounds  differing 
generally  in  character  from  their  constituents,  is  that  mode  of 
force  of  which  the  human  mind  has  hitherto  formed  the  least 
definite  idea.  The  word  itself — affinity — is  ill  chosen,  its 
meaning,  in  this  instance,  bearing  no  analogy  to  its  ordinary 
sense  ;  and  the  mode  of  its  action  is  conveyed  by  certain  con- 
ventional expressions,  no  dynamic  theory  of  it  worthy  of 
attention  having  been  adopted.  Its  action  so  modifies  and 
alters  the  character  of  matter,  that  the  changes  it  in- 
duces have  acquired,  not  perhaps  very  logically,  a  generic 
contradistinction  from  other  material  changes,  and  we 
thus  use,  as  contradistinguished,  the  terms  physical  and 
chemical. 

The  main  distinction  between  chemical  affinity  and  physi- 
cal attraction  or  aggregation,  is  the  difference  of  character  of 
the  chemical  compound  from  its  components.  This  is,  how- 
ever, but  a  vague  line  of  demarcation  ;  in  many  cases,  which 
would  be  classed  by  all  as  chemical  actions,  the  change  of 
character  is  but  slight ;  in  others,  as  in  the  effects  of  neutrali- 
sation, the  difference  of  character  would  be  a  result  which 
would  equally  follow  from  physical  attraction  of  dissimilar 
substances,  the  previous  characters  of  the  constituents  depend- 
ing upon  this  very  attraction  or  affinity :  thus  an  acid  corrodes 


CHEMICAL   AFFINITY.  153 

because  it  tends  to  unite  with  another  body ;  when  united, 
its  corrosive  power,  i.  e.  its  tendency  to  unite,  being  satiated, 
it  cannot,  so  to  speak,  be  further  attracted,  and  it  necessarily 
loses  its  corrosive  power.  But  there  are  other  cases  where 
no  such  result  could  d  priori  be  anticipated,  as  where  the 
attraction  or  combining  tendency  of  the  compound  is  higher 
than  that  of  its  constituents :  thus,  who  could,  by  physical 
reasoning,  anticipate  a  substance  like  nitric  acid  from  the 
combination  of  nitrogen  and  oxygen  ? 

The  nearest  approach,  perhaps,  that  we  can  form  to  a 
comprehension  of  chemical  action,  is  by  regarding  it  (vaguely 
perhaps)  as  a  molecular  attraction  or  motion.  It  will 
directly  produce  motion  of  definite  masses,  by  the  resultant 
of  the  molecular  changes  it  induces :  thus,  the  projectile 
effects  of  gunpowder  may  be  cited  as  familiar  instances  of 
motion  produced  by  chemical  action.  It  may  be  a  question 
whether,  in  this  case,  the  force  which  occasions  the  motion 
of  the  mass  is  a  conversion  of  the  force  of  chemical  affinity, 
or  whether  it  is  not,  rather,  a  liberation  of  other  forces  exist- 
ing in  a  state  of  static  equilibrium,  and  having  been  brought 
into  such  state  by  previous  chemical  actions ;  but,  at  all 
events,  through  the  medium  of  electricity  chemical  affinity 
may  be  directly  and  quantitatively  converted  into  the  other 
modes  of  force.  By  chemical  affinity,  then,  we  can  directly 
produce  electricity;  this  latter  force  was,  indeed,  said  by 
Davy  to  be  chemical  affinity  acting  on  masses :  it  appears, 
rather,  to  be  chemical  affinity  acting  in  a  definite  direction 
through  a  chain  of  particles ;  but  by  no  definition  can  the 
exact  relation  of  chemical  affinity  and  electricity  be  expressed ; 
for  the  latter,  however  closely  related  to  the  former,  yet  exists 
where  the  former  does  not,  as  in  a  metallic  wire,  which  when 
electrified,  or  conducting  electricity,  is,  nevertheless,  not 
chemically  altered,  or,  at  least,  not  known  to  be  chemically 
altered. 

Volta,  the  antitype  of  Prometheus,  first  enabled  us  de- 
7* 


154:  CORRELATION   OF   PHYSICAL  FORCES. 

finitely  to  relate  the  forces  of  chemistry  and  electricity. 
When  two  dissimilar  metals  in  contact  are  immersed  in  a 
liquid  belonging  to  a  certain  class,  and  capable  of  acting 
chemically  on  one  of  them,  what  is  termed  a  voltaic  circuit  is 
formed,  and,  by  the  chemical  action,  that  peculiar  mode  of 
force  called  an  electric  current  is  generated,  which  circulates 
from  metal  to  metal,  across  the  liquid,  and  through  the  points 
of  contact. 

Let  us  take,  as  an  instance  of  the  conversion  of  chemical 
force  into  electrical,  the  following,  which  I  made  known  some 
years  ago.  If  gold  be  immersed  in  hydrochloric  acid,  no 
chemical  action  takes  place.  If  gold  be  immersed  in  nitric 
acid,  no  chemical  action  takes  place  ;  but  mix  the  two  acids, 
and  the  immersed  gold  is  chemically  attacked  and  dissolved  : 
this  an  is  ordinary  chemical  action,  the  result  of  a  double  chemi- 
cal affinity.  In  hydrochloric  acid,  which  is  composed  of 
chlorine  and  hydrogen,  the  affinity  of  chlorine  for  gold  being 
less  than  its  affinity  for  hydrogen,  no  change  takes  place  ;  but 
when  the  nitric  acid  is  added,  this  latter  containing  a  great 
quantity  of  oxygen  in  a  state  of  feeble  combination,  the 
affinity  of  oxygen  for  hydrogen  opposes  that  of  hydrogen  for 
chlorine,  and  then  the  affinity  of  the  latter  for  gold  is  enabled 
to  act,  the  gold  combines  with  the  chlorine,  and  chloride  of 
gold  remains  in  solution  in  the  liquid.  Now,  in  order  to 
exhibit  this  chemical  force  in  the  form  of  electrical  force, 
instead  of  mixing  the  liquids,  place  them  in  separate  vessels 
or  compartments,  but  so  that  they  may  be  in  contact,  which 
may  be  effected  by  having  a  porous  material,  such  as  un- 
glazed  porcelain,  amianthus,  &c.,  between  them.  Immerse  in 
each  of  these  liquids  a  strip  or  wire  of  gold :  as  long  as  these 
pieces  of  gold  remain  separated,  no  chemical  or  electrical 
effect  takes  place ;  but  the  instant  they  are  brought  into 
metallic  contact,  either  immediately  or  by  connecting  each 
with  the  same  metallic  wire,  chemical  action  takes  place — 
the  gold  in  the  hydrochloric  acid  is  dissolved,  electrical  action 


CHEMICAL   AFFINITY.  155 

also  takes  place,  the  nitric  acid  is  deoxidised  by  the  trans- 
ferred hydrogen,  and  a  current  of  electricity  may  be  detected 
in  the  metals  or  connecting  metal  by  the  application  of  a  gal- 
vanometer or  any  instrument  appropriate  for  detecting  such 
effect. 

There  are  few,  if  any,  chemical  actions  which  cannot  be 
experimentally  made  to  produce  electricity :  the  oxidation  of 
metals,  the  burning  of  combustibles,  the  combination  of  oxy- 
gen and  hydrogen,  &c.,  may  all  be  made  sources  of  elec- 
tricity. The  common  mode  in  which  the  electricity  of  the 
voltaic  battery  is  generated  is  by  the  chemical  action  of  water 
upon  zinc ;  this  action  is  increased  by  adding  certain  acids  to 
the  water,  which  enable  it  to  act  more  powerfully  upon  the 
zinc,  or  in  some  cases  act  themselves  upon  it ;  and  one  of  the 
most  powerful  chemical  actions  known — that  of  nitric  acid 
upon  oxidable  metals — is  that  which  produces  the  most  pow- 
erful voltaic  battery,  a  combination  which  I  made  known  in 
the  year  1839 :  indeed,  we  may  safely  say,  that  when  the 
chemical  force  is  utilised,  or  not  wasted,  but  all  converted  into 
electrical  force,  the  more  powerful  the  chemical  action,  the 
more  powerful  is  the  electrical  action  which  results. 

If,  instead  of  employing  manufactured  products  or  educts, 
such  as  zinc  and  acids,  we  could  realise  as  electricity  the 
whole  of  the  chemical  force  which  is  active  in  the  combustion 
of  cheap  and  abundant  raw  materials,  such  as  coal,  wood,  fat, 
&c.,  with  air  or  water,  we  should  obtain  one  of  the  greatest 
practical  desiderata,  and  have  at  our  command  a  mechanical 
power  in  every  respect  superior  in  its  applicability  to  the 
steam  engine. 

I  have  shown  that  the  flame  of  the  common  blowpipe  gives 
rise  to  a  very  marked  electrical  current,  capable  not  only  of 
affecting  the  galvanometer,  but  of  producing  chemical  decom- 
position :  two  plates  or  coils  of  platinum  are  placed,  the  one 
in  the  portion  of  the  flame  near  the  orifice  of  the  jet,  or  at 
the  points  where  combustion  commences,  the  other  in  the  full 


156  CORRELATION   OF  PHYSICAL   FORCES. 

yellow  flame  where  combustion  is  at  its  maximum  ;  this  latter 
should  be  kept  cool,  to  enable  a  thermo-electric  current,  which 
is  produced  by  the  different  temperature  of  the  platinum 
plates,  to  co-operate  with  the  flame  current ;  wires  attached 
to  the  plates  of  platinum  form  the  terminals  or  poles.  By  a 
row  of  jets  a  flame  battery  may  be  formed,  yielding  increased 
effects  ;  but  in  these  experiments,  though  theoretically  inter- 
esting, so  small  a  fraction  of  the  power,  actually  at  work  in 
the  combustion,  has  been  thrown  into  an  electrical  form,  that 
there  is  no  immediate  promise  of  a  practical  result. 

The  quantity  of  the  electrical  current,  as  measured  by  the 
quantity  of  matter  it  acts  upon  in  its  different  phenomenal 
effects,  is  proportionate  to  the  quantity  of  chemical  action 
which  generated  it ;  and  its  intensity,  or  power  of  overcoming 
resistance,  is  also  proportionate  to  the  intensity  of  chemical 
affinity  when  a  single  voltaic  pair  is  employed,  or  to  the  num- 
ber of  reduplications  when  the  well-known  instrument  called 
the  voltaic  battery  is  used. 

The  mode  in  which  the  voltaic  current  is  increased  in  in- 
tensity by  these  reduplications,  is  in  itself  a  striking  instance 
of  the  mutual  relations  and  dynamic  analogies  of  different 
forces.  Let  a  plate  of  zinc  or  other  metal  possessing  a  strong 
aifinity  for  oxygen,  and  another  of  platinum  or  other  metal 
possessing  little  or  no  affinity  for  oxygen,  be  partially  im- 
mersed in  a  vessel,  A,  containing  dilute  nitric  acid,  but  not 
in  contact  with  each  other ;  let  platinum  wires  touching  each 
of  these  plates  have  their  extremities  immersed  in  another 
vessel,  B,  containing  also  dilute  nitric  acid :  as  the  acid  in 
vessel  A  is  decomposed,  by  the  chemical  affinity  of  the  zinc 
for  the  oxygen  of  the  acid,  the  acid  in  vessel  B  is  also  decom- 
posed, oxygen  appearing  at  the  extremity  of  the  wire  which 
is  connected  with  the  platinum :  the  chemical  power  is  con- 
veyed or  transferred  through  the  wires,  and,  abstracting  cer- 
tain local  effects,  for  every  unit  of  oxygen  which  combines 
with  the  zinc  in  the  one  vessel,  a  unit  of  oxygen  is  evolved 


CHEMICAL   AFFINITY.  157 

from  the  platinum  wire  in  the  other.  The  platinum  wire  is 
thus  thrown  into  a  condition  analogous  to  zinc,  or  has  a  pow 
er  given  to  it  of  determining  the  oxygen  of  the  liquid  to  its 
surface,  though  it  cannot,  as  is  the  case  with  zinc,  com- 
bine with  it  under  similar  circumstances.  If  we  now  substi- 
tute for  the  platinum  wire  which  was  connected  with  the 
platinum  plate,  a  zinc  wire,  we  have  in  addition  to  the  deter- 
mining tendency  by  which  the  platinum  was  affected,  the 
chemical  affinity  of  the  oxygen  in  vessel  B  for  the  zinc  wire  : 
thus  we  have,  added  to  the  force  which  was  originally  pro- 
duced by  the  zinc  of  the  combination  in  vessel  A,  a  second 
force,  produced  by  the  zinc  in  vessel  B,  co-operating  with  the 
first ;  two  pairs  of  zinc  and  platinum  thus  connected  produce, 
therefore,  a  more  intense  effect  than  one  pair ;  and  if  we  go 
on  adding  to  these  alternations  of  zinc,  platinum,  and  liquid, 
we  obtain  an  indefinite  exaltation  of  chemical  power,  just  as 
in  mechanics  we  obtain  accelerated  motion  by  adding  fresh 
impulses  to  motion  already  generated. 

The  same  rule  of  proportion  which  holds  good  in  chemi- 
cal combinations  also  obtains  in  electrical  effects,  when  these 
are  produced  by  chemical  actions.  Dalton  and  others  proved 
that  the  constituents  of  a  vast  number  of  compound  substances 
always  bore  a  definite  quantitative  relation  to  each  other : 
thus,  water,  which  consists  of  one  part  by  weight  of  hydro- 
gen united  to  eight  parts  of  oxygen,  cannot  be  formed  by  the 
same  elements  in  any  other  than  these  proportions  ;  you  can 
neither  add  to  nor  subtract  from  the  normal  ratio  of  the 
elements,  without  entirely  altering  the  nature  of  the  com- 
pound. Further,  if  any  element  be  selected  as  unity,  the 
combining  ratios  of  other  elements  will  bear  an  invariable 
quantitative  relation  to  that  and  to  each  other :  thus  if  hydro- 
gen be  chosen  as  1,  oxygen  will  be  8,  chlorine  will  be  36  ; 
that  is,  oxygen  will  unite  with  hydrogen  in  the  proportion  of 
8  parts  by  weight  to  1,  while  chlorine  will  unite  with  hydro- 
gen in  the  proportion  of  36  to  1,  or  with  oxygen  in  the  pro- 


158  CORRELATION   OF   PHYSICAL  FORCES. 

portion  of  36  to  8.  Numbers  expressing  their  combining 
weights,  which  are  thus  relative,  not  absolute,  may  by  a  con- 
ventional assent  as  to  the  point  of  unity,  be  fixed  for  all  chemi- 
cal reagents  ;  and,  when  so  fixed,  it  will  be  found  that  bodies, 
at  least  in  inorganic  compounds,  generally  unite  in  those  pro- 
portions, or  in  simple  multiples  of  them :  these  proportions 
are  termed  Equivalents. 

„  Now  a  voltaic  battery,  which  consists  usually  of  alterna- 
tions of  two  metals,  and  a  liquid  capable  of  acting  chemically 
upon  one  of  them,  has,  as  we  have  seen,  the  power  of  pro- 
ducing chemical  action  in  a  liquid  connected  with  it  by  metals 
upon  which  this  liquid  is  incapable  of  acting :  in  such  case  the 
constituents  of  the  liquid  will  be  eliminated  at  the  surfaces  of 
the  immersed  metals,  and  at  a  distance  one  from  the  other. 
For  example,  if  the  two  platinum  terminals  of  a  voltaic 
battery  be  immersed  in  water,  oxygen  will  be  evolved  at  one 
and  hydrogen  at  the  other  terminal,  exactly  in  the  propor- 
tions in  which  they  form  water ;  while,  to  the  most  minute 
examination,  no  action  is  perceptible  in  the  stratum  of 
liquid.  It  was  known  before  Faraday's  time  that,  while  this 
chemical  action  was  going  on  in  the  subjected  liquid,  a  chemi- 
cal action  was  going  on  in  the  cells  of  the  voltaic  battery ; 
but  it  was  scarcely  if  at  all  known  that  the  amount  of  chemi- 
cal action  in  the  one  bore  a  constant  relation  to  the  amount 
of  action  in  the  other.  Faraday  proved  that  it  bore  a  direct 
equivalent  relation :  that  is,  supposing  the  battery  to  be 
formed  of  zinc,  platinum,  and  water,  the  amount  of  oxygen 
which  united  with  the  zinc  in  each  cell  of  the  battery  was 
exactly  equal  to  the  amount  evolved  at  the  one  platinum  ter- 
minal, while  the  hydrogen  evolved  from  each  platinum  plate 
of  the  battery  was  equal  to  the  hydrogen  evolved. from  the 
other  platinum  terminal. 

Supposing  the  battery  to  be  charged  with  hydrochloric 
acid,  instead  of  water,  while  the  terminals  are  separated  by 

water,  then  for  every  36  parts  by  weight  of  chlorine  which 


CHEMICAL   AFFINITY.  159 

united  with  each  plate  of  zinc,  eight  parts  of  oxygen  would 
be  evolved  from  one  of  the  platinum  terminals  :  that  is,  the 
weights  would  be  precisely  in  the  same  relation  which  Dalton 
proved  to  exist  in  their  chemical  combining  weights.  This 
may  be  extended  to  all  liquids  capable  of  being  decomposed 
by  the  voltaic  force,  thence  called  Electrolytes :  and  as  no  vol- 
taic effect  is  produced  by  liquids  incapable  of  being  thus  de- 
composed, it  follows  that  voltaic  action  is  chemical  action  tak- 
ing place  at  a  distance,  or  transferred  through  a  chain  of 
media,  and  that  the  chemical  equivalent  numbers  are  the  ex- 
ponents of  the  amount  of  voltaic  action  for  corresponding 
chemical  substances. 

As  heat,  light,  magnetism,  or  motion,  can  be  produced  by 
the  requisite  application  of  the  electric  current,  and  as  this  is 
definitely  produced  by  chemical  action,  we  get  these  forces 
very  definitely,  though  not  immediately,  produced  by  chemi- 
cal action.  Let  us,  however,  here  enquire,  as  we  have  al- 
ready done  with  respect  to  the  other  forces,  how  far  other 
forces  may  directly  emanate  from  chemical  affinity. 

Heat  is  an  immediate  product  of  chemical  affinity.  I 
know  of  no  exception  to  the  general  proposition  that  all  bod- 
ies in  chemically  combining  produce  heat ;  i.  e.  if  solu- 
tion be  not  considered  as  chemical  action,  and  even  in  that 
case,  when  cold  results,  it  is  from  a  change  of  consistence,  as 
from  the  solid  to  the  liquid  state,  and  not  from  chemical 
action. 

We  shall  find  that  the  same  view  of  the  expenditure  of 
force  which  we  have  considered  in  treating  of  latent  heat 
holds  good  as  to  the  expenditure  of  chemical  force  when  re- 
garded with  reference  to  the  amount  of  heat  or  repulsive 
force  which  it  engenders,  the  chemical  force  being  here  ex- 
hausted by  chemical  expansion — that  is,  by  heat.  Thus,  in 
the  chemical  action  of  the  ordinary  combustion  of  coal  and 
oxygen,  the  expenditure  of  fuel  will  be  in  proportion  to  the 
expansibility  of  the  substances  heated ;  water  passing  freely 


160  COEKELATION   OF  PHYSICAL   FOBCE9. 

into  the  steam  will  consume  more  fuel  than  if  it  be  confined 
and  kept  at  a  temperature  above  its  boiling  point. 

Why  chemical  action  produces  heat,  or  what  is  the  action 
of  the  molecules  of  matter  when  chemically  uniting,  is  a 
question  upon  which  many  theories  have  been  proposed  and 
which  may  possibly  be  never  more  than  approximately  re- 
solved. 

Some  authors  explain  it  by  the  condensation  which  takes 
place  ;  but  this  will  not  account  for  the  many  instances  where, 
from  the  liberation  of  gases,  a  great  increase  of  volume  en- 
sues upon  chemical  combustion,  as  in  the  familiar  instance 
of  the  explosion  of  gunpowder :  others  explain  it  as  resulting 
from  the  union  of  atmospheres  of  positive  and  negative  elec- 
tricity which  are  assumed  to  surround  the  atoms  of  bodies ; 
but  this  involves  hypothesis  upon  hypothesis.  Dr.  Wood  has 
lately  thrown  out  a  view  of  the  heat  of  chemical  action  which 
is  more  in  accordance  with  a  dynamic  theory  of  heat,  and 
as  such  demands  some  notice.  Starting  with  his  proposition, 
which  I  have  previously  mentioned,  ;  that  the  nearer  the  par- 
ticles of  bodies  are  to  each  other  the  less  they  require  to 
move  to  produce  a  given  motion  in  the  particles  of  another 
body/  his  argument,  if  I  rightly  understand  it,  assumes  some- 
thing of  this  form. 

In  the  mechanical  approximation  of  the  particles  of  a 
homogeneous  body  heat  results  ;  the  particles  a  a  of  the  body 
A  would,  by  their  approximation,  produce  expansion  in  the 
neighbouring  body  B,  the  more  so  in  proportion  as  they  them- 
selves were  previously  nearer  to  each  other.  In  chemically 
combining,  a  a  the  particles  of  A  are  brought  into  very  close 
proximity  with  b  b  the  particles  of  B  ;  heat  should  therefore 
result,  and  the  greater  because  the  proximity  may  fairly  be 
assumed  to  be  greater  in  the  case  of  chemical  combination 
than  in  that  of  mechanical  compression.  In  cases,  then, 
where  there  is  no  absolute  diminution  of  bulk  ensuing  on 
chemical  combination,  if  the  greater  proximity  of  the  com- 


CHEMICAL  AFFINITY.  161 

bining  particles  be  such  that  the  correlative  expansion  ought 
to  be  greater  (if  there  were  no  chemical  combination)  than 
that  occupied  by  the  total  volume  of  the  new  compound,  an 
extra  expanding  power  is  evolved,  and  heat  or  expansion 
ought  to  be  produced  in  surrounding  bodies.  In  other  words, 
if  a  a  could  be  brought  by  physical  attraction  as  near  each 
other  as  they  are  by  chemical  attraction  brought  near  to  b  5, 
they  would,  from  their  increased  proximity,  produce  an  ex- 
pansive power  ultra  the  volume  occupied  by  the  actual  chem- 
ical compound  A  and  B.  The  question,  however,  immedi- 
ately occurs,  why  should  the  volume  of  the  compound  be  lim- 
ited and  not  occupy  the  full  space  equivalent  to  the  expanding 
power  induced  by  the  contraction  or  approximation  of  the 
particles.  As  the  distance  of  the  particles  is  the  resultant 
of  the  contending  contracting  and  expanding  powers,  this 
result  ought  to  express  itself  in  terms  of  the  actual  volume 
produced  by  the  combination,  which  it  certainly  does  not. 

Though  I  see  some  difficulties  in  Dr.  Wood's  theory,  and 
perhaps  have  not  rightly  conceived  it,  his  views  have  to  my 
mind  great  interest,  his  mode  of  regarding  natural  phenomena 
being  analogous  to  that  which  I  have  in  this  Essay,  and  for 
many  years,  advocated,  viz.  to  divest  physical  science  as 
much  as  possible  of  hypothetic  fluids,  ethers,  latent  entities, 
occult  qualities,  &c.  My  own  notion  of  the  heat  produced 
by  chemical  combination,  though  I  scarcely  dare  venture  an 
opinion  upon  a  subject  so  controverted,  is,  that  it  is  analogous 
to  the  heat  of  friction,  that  the  particles  of  matter  in  close 
approximation  and  rapid  motion  inter  se  evolve  heat  as  a  con- 
tinuation of  the  motion  interrupted  by  the  friction  or  intesti- 
nal motion  of  the  particles :  heat  would  thus  be  produced, 
whether  the  resulting  compound  were  of  greater  or  less  bulk 
than  the  sum  of  the  components,  though  of  course  when  the 
compound  is  of  greater  bulk  less  heat  would  be  apparent  in 
neighbouring  bodies,  the  expansion  taking  place  in  one  of  the 
substances  themselves — I  say  in  one  of  them,  for  it  is  stated 


162  CORRELATION   OF   PHYSICAL   FORCES. 

in  books  of  authority  that  there  is  no  instance  of  two  or  more 
solids  or  liquids,  or  a  solid  and  a  liquid,  combining  and  pro- 
ducing a  compound  which  is  entirely  gaseous  at  ordinary 
temperatures  and  pressures.  The  substance  gun-cotton,  how- 
ever, discovered  by  Dr.  Schoenbein,  very  nearly  realises  this 
proposition. 

Dr.  Andrews  has  arrived  at  the  conclusion,  after  careful 
experiment,  that  in  chemical  combinations  where  acids  and 
alkalies  or  analogous  substances  are  employed,  the  amount  of 
heat  produced  is  determined  by  the  basic  ingredient,  and  his 
experiments  have  received  general  assent ;  although  it  should 
be  stated  that  M.  Hess  arrived  at  contrary  results,  the  acid 
constituent  according  to  his  experiments  furnishing  the  meas- 
ure of  the  heat  developed. 

Light  is  directly  produced  by  chemical  action,  as  in  the  flash 
of  gunpowder,  the  burning  of  phosphorus  in  oxygen  gas,  and  all 
rapid  combustions :  indeed,  wherever  intense  heat  is  developed, 
light  accompanies  it.  In  many  cases  of  slow  combustion, 
such  as  the  phenomena  of  phosphorescence,  the  light  is  apparent- 
ly much  more  intense  than  the  heat ;  the  former  being  obvious, 
the  latter  so  difficult  of  detection  that  for  a  long  time  it  was 
a  question  whether  any  heat  was  eliminated ;  and  I  am  not 
aware  that  at  the  present  day,  any  thermic  effects  from  cer- 
tain modes  of  phosphorescence,  such  as  those  of  phospho- 
rescent wood,  putrescent  fish,  &c.,  have  been  detected. 

Chemical  action  produces  magnetism  whenever  it  is  thrown 
into  a  definite  direction,  as  in  the  phenomenon  of  electrolysis. 
I  may  adduce  the  gas  voltaic  battery,  as  presenting  a  simple 
instance  of  the  direct  production  of  magnetism  by  chemical 
synthesis.  Oxygen  and  hydrogen  in  that  combination  chemi- 
cally unite  ;  but  instead  of  combining  by  intimate  molecular 
admixture,  as  in  the  ordinary  cases,  they  act  upon  water,  i.  e. 
combined  oxygen  and  hydrogen,  placed  between  them  so  as 
to  produce  a  line  of  chemical  action  ;  and  a  magnet  adjacent 
to  this  line  of  action  is  deflected,  and  places  itself  at  right 


CHEMICAL   AFFINITY.  163 

angles  to  it.  What  a  chain  of  molecules  does  here,  there 
can  be  no  doubt  all  the  molecules  entering  into  combination 
would  produce  in  ordinary  chemical  actions ;  but  in  such 
cases,  the  direction  of  the  lines  of  combination  being  irregu- 
lar and  confused,  there  is  no  general  resultant  by  which  the 
magnet  can  be  affected. 

What  the  exact  nature  of  the  transference  of  chemical 
power  across  an  electrolyte  is,  we  at  present  know  not,  nor 
can  we  form  any  more  definite  idea  of  it  than  that  given  by 
the  theory  of  Grotthus.  We  have  no  knowledge  as  to  the 
exact  nature  of  any  mode  of  chemical  action,  and,  for  the 
present  must  leave  it  as  an  obscure  action  of  force,  of  which 
future  researches  may  simplify  our  apprehension. 

We  have  seen  that  an  equivalent  or  proportionate  electri- 
cal effect  is  produced  by  a  given  amount  of  chemical  action ; 
if  we,  in  turn,  produce  heat  and  magnetism  and  motion  by 
the  electricity  resulting  from  chemical  action,  we  shall  be  able 
to  measure  these  forces  far  more  accurately  than  when  they 
are  directly  produced,  and  thus  to  deduce  their  equivalent  re- 
lation to  the  initial  chemical  action.  Thus  M.  Favre,  after 
ascertaining  the  quantity  of  heat  produced  by  the  oxidation 
of  a  quantity  of  zinc,  and  finding,  as  have  others,  that  the 
heat  is  the  same  when  evolved  from  a  voltaic  battery  by  the 
same  consumption  of  zinc  forming  its  positive  element,  makes 
the  following  experiment. 

A  voltaic  battery  and  electro-magnet  are  immersed  in  cal- 
orimeters, and  the  heat  produced  when  the  connection  with 
the  magnet  is  effected  is  noted. 

The  electro-magnet  is  then  made  to  raise  a  weight,  and 
thus  perform  mechanical  work,  and  the  heat  produced  is 
again  noted.  It  is  found  in  the  latter  case  that  less  heat  is 
evolved  than  in  the  former,  a  certain  quantity  of  heat  has 
therefore  been  replaced  by  the  mechanical  work ;  and  by  esti- 
mating the  amount  of  heat  subtracted,  and  the  amount  of 
work  produced,  he  deduces  the  relative  equivalent  of  work  to 


164  CORRELATION   OF  PHYSICAL   FORCES. 

heat.  These  experiments  give  a  production  of  mechanical 
work  by  chemical  action,  not,  it  is  true,  a  direct  production, 
but,  as  the  heat  and  work  are  in  inverse  ratios,  and  each  has 
its  source  in  chemical  action,  they  prove  that  they  are  definite 
for  a  definite  amount  of  chemical  action,  and  as  each  is  pro- 
duced respectively  by  electricity  and  magnetism,  these  forces 
must  also  bear  a  definite  relation  to  the  initial  chemical  force. 

The  doctrine  of  definite  combining  proportions,  which  so 
beautifully  serves  to  relate  chemistry  to  voltaic  electricity, 
led  to  the  atomic  theory,  which,  though  adopted  in  its  univer- 
sality by  a  large  majority  of  chemists,  presents  great  difficul- 
ties when  extended  to  all  chemical  combinations. 

The  equivalent  ratios  in  which  a  great  number  of  sub- 
stances chemically  combine,  hold  good  in  so  many  instances, 
that  the  atomic  doctrine  is  believed  by  many  to  be  universally 
applicable,  and  called  a  law ;  and  yet,  when  followed  in  the 
combinations  of  substances  whose  natural  chemical  attractions 
are  very  feeble,  the  relation  fades  away,  and  is  sought  to  be 
recovered  by  applying  a  separate  and  arbitrary  multiplier  to 
the  different  constituents. 

Thus,  when  it  was  found  that  a  vast  number  of  substances 
combined  in  definite  volumes  and  weights,  and  in  definite  vol- 
umes and  weights  only,  it  was  argued  that  their  ultimate 
molecules  or  atoms  had  a  definite  size,  as  otherwise  there 
was  no  apparent  reason  why  this  equivalent  ratio  should  hold 
good  :  why,  for  instance,  water  should  only  be  formed  of  two 
volumes  or  one  unit  by  weight  of  hydrogen,  and  of  one  vol- 
ume or  eight  units  by  weight  of  oxygen ;  why,  unless  there 
were  some  ultimate  limits  to  the  divisibility  of  its  molecules, 
should  not  water,  or  a  fluid  substance  approximating  to  water 
in  character,  be  formed  by  a  half,  a  third,  or  a  tenth  part  of 
hydrogen,  with  eight  parts  of  oxygen  ? 

It  was  perfectly  consistent  with  the  atomic  view  that  a 
substance  might  be  formed  with  one  part  combined  with  eight 
parts,  or  with  sixteen,  or  with  twenty-four,  for  in  such  a  sub- 


CHEMICAL  AFFINITY.  165 

stance  there  would  be  no  subdivision  of  the  (supposed  indivi- 
sible) molecule  ;  and  this  held  good  with  many  compounds  : 
thus  fourteen  parts  by  weight,  say  grains  of  nitrogen,  will 
combine  respectively  with  eight,  sixteen,  twenty-four,  thirty- 
two,  and  forty  parts  by  weight,  or  grains,  of  oxygen. 

So,  again,  twenty-seven  grains  of  iron  will  combine  with 
eight  grains  of  oxygen  or  with  twenty-four  grains,  i.  e.  three 
proportionals  of  oxygen.  No  compound  is  known  in  which 
twenty-seven  grains  of  iron  will  combine  with  two  propor- 
tionals or  sixteen  grains  of  oxygen ;  but  this  does  not  much 
affect  the  theory,  as  such  a  compound  may  be  yet  discovered, 
or  there  may  be  reasons  at  present  unknown  why  it  cannot  be 
formed. 

But  now  comes  a  difficulty :  twenty-seven  parts  by  weight 
of  iron  will  combine  with  twelve  parts  by  weight  of  oxygen, 
and  twenty-seven  parts  of  iron  will  also  combine  with  ten 
and  two-third  parts  of  oxygen.  Thus  if  we  retain  the  unit 
of  iron  we  must  subdivide  the  unit  of  oxygen,  or  if  we  retain 
the  unit  of  oxygen  we  must  subdivide  the  unit  of  iron,  or  we 
must  subdivide  both  by  a  different  divisor.  What  then  be- 
comes of  the  notion  of  an  atom  or  molecule  physically  indi- 
visible ?  / 

If  iron  were  the  only  substance  to  which  this  difficulty 
applied,  it  might  be  viewed  as  an  unexplained  exception,  or 
as  a  mixture  of  two  oxides  ;  or  recourse  might  be  had  to  a 
more  minute  subdivision  to  form  the  units  or  equivalents  of 
other  substances  ;  but  numerous  other  substances  fall  under 
a  similar  category ;  and  in  organic  combinations,  to  preserve 
the  atomic  nomenclature  we  must  apply  a  separate  multiplier 
or  divisor  to  far  the  greater  number  of  the  elementary  con- 
stituents, i.  e.  we  must  divide  that  which  is,  ex  hypothesi,  indi- 
visible. 

Thus,  to  take  a  more  complex  substance  than  any  formed 
by  the  combination  of  iron  and  oxygen,  let  us  select  the  sub- 
stance albumen,  composed  of  carbon,  hydrogen,  nitrogen, 


166  CORRELATION  OF  PHYSICAL  FORCES. 

oxygen,  phosphorus,  and  sulphur.  In  this  case  we  must  ei- 
ther divide  the  atoms  of  phosphorus  and  sulphur  so  as  to  re- 
duce them  to  small  fractions,  or  multiply  the  atoms  of  the 
other  substances  by  extravagant  numbers ;  thus  to  preserve 
the  unit  of  one  of  the  constituents  of  this  substance,  chemists 
say  it  is  composed  of  400  atoms  of  carbon,  310  of  hydrogen, 
120  of  oxygen,  50  of  nitrogen,  2  of  sulphur,  and  1  of  phos- 
phorus. This  is  a  somewhat  extreme  case,  but  similar  diffi- 
culties will  be  found  in  different  degrees  to  prevail  among  or- 
ganic compounds  ;  in  very  many  no  constituent  can  be  taken 
as  a  unit  to  which  simple  multiples  of  any  of  the  others  will 
give  their  relative  proportions.  By  the  mode  of  notation 
adopted,  if  any  conceivable  substance  be  selected,  it  could? 
whatever  be  the  proportions  of  its  constituents,  be  termed 
atomic.  A  solution  of  an  ounce  of  sugar  in  a  pound  of  wa- 
ter, in  a  pound  and  a  half,  in  a  pound  and  a  quarter,  in  a 
pound  and  a  tenth,  might  be  expressed  in  an  atomic  form,  if 
we  select  arbitrarily  a  multiplier  or  divisor. 

It  is  true  that  in  the  case  of  solution,  different  proportions 
can  be  united  up  to  the  point  of  saturation  without  any  dif- 
ference in  the  character  of  the  compound,  though  the  same 
may  be  predicated  to  some  extent  of  an  acid  and  an  alkali ; 
but  even  where  the  steps  are  sudden,  and  compounds  only 
exist  with  definite  proportions,  they  cannot,  in  a  multitude  of 
cases,  be  reconciled  with  the  true  idea  of  an  atomic  combina- 
tion, i.  e.  one  to  one,  one  to  two,  &c. 

Although,  therefore,  nature  presents  us  with  facts  which 
show  that  there  is  some  restrictive  law  of  combination  which 
in  numerous  cases  limits  the  ratios  in  which  substances  will 
combine,  nay,  further,  shows  many  instances  of  a  proportion 
between  the  combining  weights  of  one  compound  and  those 
of  another ;  although  she  shows  also  a  remarkable  simplicity 
in  the  combining  volumes  of  numerous  gases,  she  also  gives 
numerous  cases  to  which  the  doctrine  of  atomic  combinations 
cannot  fairly  be  applied. 


CHEMICAL   AFFINITY.  167 

That  there  must  be  something  in  the  constitution  of  mat- 
ter, or  in  the  forces  which  act  on  it,  to  account  for  the  per 
saltum  manner  in  which  chemical  combinations  take  place,  is 
inevitable  ;  but  the  idea  of  atoms  does  not  seem  satisfactorily 
to  account  for  it. 

By  selecting  a  separate  multiplier  or  divisor,  chemists 
may  denote  every  combination  in  terms  derived  from  the 
atomic  theory ;  but  they  have  passed  from  the  original  law, 
which  contemplated  only  definite  multiples,  and  the  very  hy- 
pothetic expressions  of  atoms,  which  the  apparently  simple 
relations  of  combining  weights  first  led  them  to  adopt,  they 
are  obliged  to  vary  and  to  contradict  in  terms,  by  dividing 
that  which  their  hypothesis  and  the  expression  of  it  assumed 
to  be  indivisible. 

While,  therefore,  I  fully  recognise  a  great  natural  truth 
in  the  definite  ratios  presented  by  a  vast  number  of  chemical 
combinations,  and  in  the  per  saltum  steps  in  which  nearly  all 
take  place,  I  cannot  accept  as  an  argument  in  favour  of  an 
atomic  theory,  those  combinations  which  are  made  to  support 
it  by  the  application  of  an  arbitrary  notation. 

A  similar  straining  of  theory  seems  gradually  obtaining 
in  regard  to  the  doctrine  of  compound  radicals.  The  discov- 
ery of  cyanogen  by  Gay-Lussac  was  probably  the  first  in- 
ducement to  the  doctrine  of  compound  radicals ;  a  doctrine 
which  is  now  generally,  perhaps  too  generally,  received  in 
organic  chemistry.  As,  in  the  case  of  cyanogen,  a  body  ob- 
viously compound  discharged  in  almost  all  its  reactions  the 
functions  of  an  element,  so  in  many  other  cases  it  was  found 
that  compound  bodies  in  which  a  great  number  of  elements 
existed,  might  be  regarded  as  binary  combinations,  by  con- 
sidering certain  groups  of  these  elements  as  a  compound  rad- 
ical ;  that  is,  as  a  simple  body  when  treated  of  in  relation  to 
the  other  complex  substances  of  which  it  forms  part,  and 
only  as  non-elementary  when  referred  to  its  internal  consti- 
tution. 


168  CORRELATION   OF   PHYSICAL  FORCES. 

Undoubtedly,  by  approximating  in  theory  the  reactions  of 
inorganic  and  of  organic  chemistry,  by  keeping  the  mind 
within  the  limits  of  a  beaten  path,  instead  of  allowing  it  to 
wander  through  a  maze  of  isolated  facts,  the  doctrine  of  com- 
pound radicals  has  been  of  service  ;  but,  on  the  other  hand, 
the  indefinite  variety  of  changes  which  may  be  rung  upon  the 
composition  of  an  organic  substance,  by  different  associations 
of  its  primary  elements,  makes  the  binary  constituents  vary 
as  the  minds  of  the  authors  who  treat  of  them,  and  makes 
their  grouping  depend  entirely  upon  the  strength  of  the  anal- 
ogies presented  to  each  individual  mind.  From  this  cause, 
and  from  the  extreme  license  which  has  been  taken  in  theo- 
retic groupings  deduced  from  this  doctrine,  a  serious  question 
arises  whether  it  may  not  ultimately,  unless  carefully  re- 
stricted, produce  confusion  rather  than  simplicity,  and  be  to 
the  student  an  embarrassment  rather  than  an  assistance. 


VIII.—  OTHEK  MODES  OF  FOEOE. 


,  or  the  chemical  action  induced  by  the 
mere  presence  of  a  foreign  body,  embraces  a  class  of 
facts  which  must  considerably  modify  many  of  our  notions  of 
chemical  action  :  thus  oxygen  and  hydrogen,  when  mixed  in 
a  gaseous  state,  will  remain  unaltered  for  an  indefinite  pe- 
riod ;  but  the  introduction  to  them  of  a  slip  of  clean  plati- 
num will  cause  more  or  less  rapid  combination,  without  being 
in  itself  in  any  respect  altered.  On  the  other  hand,  oxygen- 
ated water,  which  is  a  compound  of  one  equivalent  of  hydro- 
gen plus  two  of  oxygen,  will,  when  under  a  certain  tempera- 
ture, remain  perfectly  stable  ;  but  touch  it  with  platinum  in 
a  state  of  minute  division,  and  it  is  instantly  decomposed, 
one  equivalent  of  oxygen  being  set  free.  Here,  again,  the 
platinum  is  unaltered,  and  thus  we  have  synthesis  and  analy- 
sis effected  apparently  by  the  mere  contact  of  a  foreign  body, 
It  is  not  improbable  that  the  increased  electrolytic  power  of 
water  by  the  addition  of  some  acids,  such  as  the  sulphuric 
and  phosphoric,  where  the  acids  themselves  are  not  decom- 
posed, depends  upon  a  catalytic  effect  of  these  acids  ;  but  we 
know  too  little  of  the  nature  and  rationale  of  catalysis  to  ex- 
press any  confident  opinion  on  its  modes  of  action,  and  pos- 
sibly we  may  comprehend  very  different  molecular  actions 
under  one  and  the  same  name.  In  no  case  does  catalysis 
yield  us  new  power  or  force  :  it  only  determines  or  facilitates 
8 


170  CORRELATION  OF   PHYSICAL   FORCES. 

the  action  of  chemical  force,  and,  therefore,  is  no  creation  of 
force  by  contact. 

The  force  so  developed  by  catalysis  may  be  converted 
into  a  voltaic  form  thus :  in  a  single  pair  of  the  gas  battery 
above  alluded  to,  one  portion  of  a  strip  of  platinum  is  im- 
mersed in  a  tube  of  oxygen,  the  other  in  one  of  hydrogen, 
both  the  gases  and  the  extremities  of  the  platinum  being  con- 
nected by  water  or  other  electrolyte  ;  a  voltaic  combination 
is  thus  formed,  and  electricity,  heat,  light,  magnetism,  and 
motion,  produced  at  the  will  of  the  experimenter. 

In  this  combination  we  have  a  striking  instance  of  cor- 
relative expansions  and  contractions,  analogous,  though  in  a 
much  more  refined  form,  to  the  expansions  and  contractions 
by  heat  and  cold  detailed  in  the  early  part  of  this  essay,  and 
illustrated  by  the  alternations  of  two  bladders  partially  filled 
with  air :  thus,  as  by  the  effect  of  chemical  combination  in 
each  pair  of  tubes  of  the  gas  battery  the  gases  oxygen  and 
hydrogen  lose  their  gaseous  character  and  shrink  into  water, 
so  at  the  platinum  terminals  of  the  battery,  when  immersed 
in  water,  water  is  decomposed,  and  expands  into  oxygen  and 
hydrogen  gases.  The  correlate  of  the  force  which  changes 
gas  into  liquid  at  one  point  of  space,  changes  liquid  into  gas 
at  another,  and  the  exact  volume  which  disappears  in  the 
one  place  reappears  in  the  other ;  so  that  it  would  appear  to 
an  inexperienced  eye  as  though  the  gases  passed  through 
solid  wires. 

Gravitation,  inertia,  and  aggregation,  were  but  cursorily 
alluded  to  in  my  original  lectures  ;  their  relation  to  the  other 
modes  of  force  seemed  to  be  less  definitely  traceable  ;  but  the 
phenomenal  effects  of  gravitation  and  inertia,  being  motion 
and  resistance  to  motion,  in  considering  motion  I  have  in 
some  degree  included  their  relations  to  the  other  forces. 

To  my  mind  gravitation  would  only  produce  other  force 
when  the  motion  caused  by  it  ceases.  Thus,  if  we  suppose 
a  meteor  to  be  a  mass  rotating  in  an  orbit  round  the  earth, 


OTHEE  MODES  OF  FOECE.  171 

and  with  no  resisting  medium,  then,  as  long  as  that  rotation 
continues,  the  motion  of  the  meteoric  mass  itself  would  be 
the  exponent  of  the  force  impelling  it ;  if  there  be  a  resist- 
ing medium,  part  of  this  motion  would  be  arrested  and  taken 
up  by  the  medium,  either  as  motion,  heat,  electricity,  or  some 
other  mode  of  force  ;  if  the  meteor  approach  the  earth  suffi- 
ciently to  fall  upon  it,  the  perceptible  motion  of  the  meteor 
is  stopped,  but  is  taken  up  by  the  earth  which  vibrates 
through  its  mass ;  part  also  reappears  as  heat  in  both  earth 
and  meteor,  and  part  in  the  change  in  the  earth's  position 
consequent  on  its  increase  of  gravity,  and  so  on.  Gravita- 
tion is  but  the  subjective  idea,  and  its  relation  to  other  modes 
of  force  seems  to  me  to  be  identical  with  that  of  pressure  or 
motion.  Thus,  when  arrested  motion  produces  heat,  it  mat- 
ters not  whether  the  motion  has  been  produced  by  a  falling 
body,  i.  e.  by  gravitation,  or  a  body  projected  by  an  explo- 
sive compound,  &c. ;  the  heat  will  be  the  same,  provided  the 
mass  and  velocity  at  the  time  of  arrest  be  the  same.  In  no 
other  sense  can  I  conceive  a  relation  between  gravitation  and 
the  other  forces,  and,  with  all  diffidence,  I  cannot  agree  with 
those  who  seek  a  more  mysterious  link. 

Mosotti  has  mathematically  treated  of  the  identity  of 
gravitation  with  cohesive  attraction,  and  Pliicker  has  recently 
succeeded  in  showing  that  crystalline  bodies  are  definitely  af- 
fected by  magnetism,  and  take  a  position  in  relation  to  the 
lines  of  magnetic  force  dependent  upon  their  optical  axis  or 
,axis  of  symmetry. 

What  is  termed  the  optic  axis  is  a  fixed  direction  through 
crystals,  in  which  they  do  not  doubly  refract  light,  and  which 
direction,  in  those  crystals  which  have  one  axis  of  figure,  or 
a  line  around  which  the  figure  is  symmetrical,  is  parallel  to 
the  axis  of  symmetry.  When  submitted  to  magnetic  influ- 
ence such  crystals  take  up  a  position,  so  that  their  optic  axis 
points  diamagnetically  or  transversely  to  the  lines  of  magnetic 
force ;  and  when,  as  is  the  case  in  some  crystals,  there  is 


172  COEKELATION   OF  PHYSICAL  FORCES. 

more  than  one  optic  axis,  the  resultant  of  these  axes  points 
diamagnetically.  The  mineral  cyanite  is  influenced  by  mag- 
netism in  so  marked  a  manner  that  when  suspended  it  will 
arrange  itself  definitely  with  reference  to  the  direction  of  ter- 
restrial magnetism,  and  may,  according  to  Pliicker,  be  used 
as  a  compass-needle. 

There  is  scarcely  any  doubt  that  the  force  which  is  con- 
cerned in  aggregation  is  the  same  which  gives  to  matter  its 
crystalline  form ;  indeed,  a  vast  number  of  inorganic  bodies, 
if  not  all,  which  appear  amorphous  are,  when  closely  exam- 
ined, found  to  be  crystalline  in  their  structure :  we  thus  get  a 
reciprocity  of  action  between  the  force  which  unites  the  mole- 
cules of  matter  and  the  magnetic  force,  and  through  the  me- 
dium of  the  latter  the  correlation  of  the  attraction  of  aggre- 
gation with  the  other  modes  of  force  may  be  established. 

I  believe  that  the  same  principles  and  mode  of  reasoning 
as  have  been  adopted  in  this  essay  might  be  applied  to  the 
organic  as  well  as  the  inorganic  world ;  and  that  muscular 
force,  animal  and  vegetable  heat,  &c.,  might,  and  at  some 
time  will,  be  shown  to  have  similar  definite  correlations  ;  but  I 
have  purposely  avoided  this  subject,  as  pertaining  to  a  depart- 
ment of  science  to  which  I  have  not  devoted  my  attention. 
I  ought,  however,  while  alluding  to  this  subject,  shortly  to 
mention  some  experiments  of  Professor  Matteucci,  communi- 
cated to  the  Royal  Society  in  the  year  1850,  by  which  it  ap- 
pears that  whatever  mode  of  force  it  be  which  is  propagated 
along  the  nervous  filaments,  this  mode  of  force  is  definitely, 
aifected  by  currents  of  electricity.  His  experiments  show 
that  when  a  current  of  positive  electricity  traverses  a  portion 
of  the  muscle  of  a  living  animal  in  the  same  direction  as  that 
in  which  the  nerves  ramify— i.  e.  a  direction  from  the  brain 
to  the  extremities — a  muscular  contraction  is  produced  in  the 
limb  experimented  on,  showing  that  the  nerve  of  motion  is 
affected ;  while,  if  the  current,  as  it  is  termed,  be  made  to 
traverse  the  muscle  in  the  reverse  direction,  or  towards  the 


OTHEK  MODES  OF  FORCE.  173 

nervous  centres,  the  animal  utters  cries,  and  exhibits  all  the 
indications  of  suffering  pain,  scarcely  any  muscular  move- 
ment being  produced ;  showing  that  in  this  case  the  nerves 
of  sensation  are  affected  by  the  electric  current,  and  therefore 
that  some  definite  polar  condition  exists,  or  is  induced,  in  the 
nerves,  to  which  electricity  is  correlated,  and  that  probably 
this  polar  condition  constitutes  nervous  agency.  There  are 
other  analogies  given  in  the  papers  of  M.  Matteucci,  and  de- 
rived from  the  action  of  the  electrical  organs  of  fishes,  which 
tend  to  corroborate  and  develope  the  same  view. 

By  an  application  of  the  doctrine  of  the  Correlation  of 
Forces,  Dr.  Carpenter  has  shown  how  a  difficulty  arising 
from  the  ordinary  notions  of  the  developement  of  an  organised 
being  from  its  germ-cell  may  be  lessened.  It  has  been 
thought  by  many  physiologists  that  the  nisus  formativus,  or 
organising  force  of  an  animal  or  vegetable  structure,  lies  dor- 
mant in  the  primordial  germ-cell.  '  So  that  the  organising 
force  required  to  build  up  an  oak  or  a  palm,  an  elephant  or  a 
whale,  is  concentrated  in  a  minute  particle  only  discernible 
by  microscopic  aid/ 

Certain  other  views  of  nearly  equal  difficulty  have  been 
propounded.  Dr.  Carpenter  suggests  the  probability  of  ex- 
traneous forces,  as  heat,  light,  and  chemical  affinity,  contin- 
uously operating  upon  the  material  germ ;  so  that  all  that  is 
required  in  this  is  a  structure  capable  of  receiving,  directing, 
and  converting  these  forces  into  those  which  tend  to  the  assim- 
.  ilation  of  extraneous  matter  and  the  definite  developement  of 
the  particular  structure.  In  proof  of  this  position  he  shows 
how  dependent  the  process  of  germ  developement  is  upon  the 
presence  and  agency  of  external  forces,  particularly  heat  and 
light,  and  how  it  is  regulated  by  the  measure  of  these  forces 
supplied  to  it. 

It  certainly  is  far  less  difficult  so  to  conceive  the  supply 
of  force  yielded  to  organised  beings  in  their  gradual  process 
of  growth,  than  to  suppose  a  store  of  dormant  or  latent  force 
pent  up  in  a  microscopic  monad. 


174  COKEELATION   OF   PHYSICAL   FORCES. 

As  by  the  artificial  structure  of  a  voltaic  battery,  chemi- 
cal actions  may  be  made  to  cooperate  in  a  definite  direction, 
so,  by  the  organism  of  a  vegetable  or  animal,  the  mode  of 
motion  which  constitutes  heat,  light,  &c.,  may,  without  extra- 
vagance, be  conceived  to  be  appropriated  and  changed  into 
the  forces  which  induce  the  absorption,  and  assimilation  of 
nutriment,  and  into  nervous  agency  and  muscular  power. 
Indications  of  similar  thoughts  may  be  detected  in  the  writings 
of  Liebig. 

Some  difficulty  in  studying  the  correlations  of  vital  with 
inorganic  physical  forces  arises  from  the  effects  of  sensation 
and  consciousness,  presenting  a  similar  confusion  to  that 
alluded  to,  when,  in  treating  of  heat,  I  ventured  to  suggest, 
that  observers  are  too  apt  to  confound  the  sensations  with  the 
phenomena.  Thus,  to  apply  some  of  the  considerations  on 
force,  given  in  the  introductory  portion  of  this  essay,  to  cases 
where  vitality  or  consciousness  intervenes.  When  a  weight 
is  raised  by  the  hand,  there  should,  according  to  the  doctrine 
of  non-creation  of  force,  have  been  somewhere  an  expenditure 
equivalent  to  the  amount  of  gravitation  overcome  in  raising 
the  weight.  That  there  is  expenditure  we  can  prove,  though 
in  the  present  state  of  science  we  cannot  measure  it.  Thus, 
prolong  the  effort,  raise  weights  for  an  hour  or  two,  the  vital 
powers  sink,  food,  i.  e.  fresh  chemical  force,  is  required  to 
supply  the  exhaustion.  If  this  supply  is  withheld  and  the 
exertion  is  continued,  we  see  the  consumption  of  force  in 
the  supervening  weakness  and  emaciation  of  the  body. 

The  consciousness  of  effort,  which  has  formed  a  topic  of 
argument  by  some  writers  when  treating  of  force,  and  is  by 
them  believed  to  be  that  which  has  originated  the  idea  of 
force,  may  by  the  physical  student  be  regarded  as  feeling  is 
in  the  phenomena  of  heat  and  cold,  viz.  a  sensation  of  the 
struggle  of  opposing  molecular  motions  in  overcoming  the 
resistance  of  the  masses  to  be  moved.  When  we  say  we  feel 
hot,  we  feel  cold,  we  feel  that  we  are  exerting  ourselves,  our 


OTHER  MODES  OF  FOKOE.  175 

expressions  are  intelligible  to  beings  who  are  capable  of  ex- 
periencing similar  sensations ;  but  the  physical  changes 
accompanying  these  sensations  are  not  thereby  explained. 
Without  pretending  to  know  what  probably  we  shall  never 
know,  the  actual  modus  agendi  of  the  brain,  nerres,  muscles, 
&c.,  we  may  study  vital  as  we  do  inorganic  phenomena,  both  by 
observation  and  experiment.  Thus,  Sir  Benjamin  Brodie  has 
examined  the  effect  of  respiration  on  animal  heat  by  inducing 
artificial  respiration  after  the  spinal  cord  has  been  severed ; 
in  which  case  he  finds  the  animal  heat  declines,  notwith- 
standing the  continuance  of  the  chemical  action  of  respiration, 
carbonic  acid  being  formed  as  usual ;  but  he  also  finds  that 
under  such  circumstances  the  struggles  or  muscular  actions 
of  the  animal  are  very  great,  and  sufficient  probably  to  ac- 
count for  the  force  eliminated  by  the  chemical  action  in 
digestion  and  respiration ;  and  Liebig,  by  measuring  the 
amount  of  chemical  action  in  digestion  and  respiration,  and 
comparing  it  with  the  labour  performed,  has  to  some  extent 
established  their  equivalent  relations. 

Mr.  Helmholtz  has  found  that  the  chemical  changes  which 
take  place  in  muscles  are  greater  when  these  are  made  to 
undergo  contractions  than  when  they  are  in  repose  ;  and  that, 
as  would  be  expected,  the  consumption  of  the  matter  of  the 
muscles,  or,  in  other  terms,  the  waste  or  excrementitious 
matter  thrown  off,  is  greater  in  the  former  than  in  the  latter 
case. 

M.  Matteucci  has  ascertained  that  the  muscles  of  recently 
killed  frogs  absorb  oxygen  and  exhale  carbonic  acid,  and  that 
when  they  are  thrown  into  a  state  of  contraction,  and  still 
more  when  they  perform  mechanical  work,  the  absorption  is 
increased ;  and  he  even  calculates  the  equivalents  of  work  so 
performed. 

M.  Beclard  finds  that  the  quantity  of  heat  produced  by 
voluntary  muscular  contraction  in  man  is  greater  when  that 
contraction  is  what  he  terms  static,  that  is,  when  it 


176  CORRELATION   OF  PHYSICAL  FORCES. 

no  external  work,  but  is  effort  alone,  .than  when  that  effort 
and  contraction  are  employed  dynamically,  so  as  to  raise  a 
weight  or  produce  mechanical  work. 

Thus,  though  we  may  see  no  present  promise  of  being 
able  to  resolve  sensations  into  their  ultimate  elements,  or  to 
trace,  physically,  the  link  which  unites  volition  with  exertion 
or  effort,  in  terms  of  our  own  consciousness  of  it,  we  may 
hope  to  approximate  the  solution  of  these  deeply  interesting 
questions. 

In  the  same  individual  the  chemical  and  physical  state  of 
the  secretions  in  the  warm  may  be  compared  with  those  in 
the  cold  parts  of  the  body.  The  changes  in  digestion  and 
respiration,  when  the  body  is  in  a  state  of  rest,  may  be  com- 
pared with  those  which  obtain  when  it  is  in  a  state  of  activity. 
The  relations  with  external  matter,  maintaining,  by  the  con- 
stant play  of  natural  forces,  the  vital  nucleus,  or  the  organi- 
sation by  means  of  which  matter  and  force  receive,  for  a 
definite  period,  a  definite  incorporation  and  direction,  may  be 
ascertained,  while  the  more  minute  structural  changes  are 
revealed  to  us  by  the  ever-improving  powers  of  the  micro- 
scope ;  and  thus  step  by  step  we  may  learn  that  which  it  is 
given  to  us  to  learn,  boundless  in  its  range  and  infinite  in  its 
progress,  and  therefore  never  giving  a  response  to  the  ultimate 
—Why? 

As  die  first  glimpse  of  a  new  star  is  caught  by  the  eye  of 
the  astronomer  while  directing  his  vision  to  a  different  point 
of  space,  and  disappears  when  steadfastly  gazed  at,  only  to 
have  its  position  and  figure  ultimately  ascertained  by  the  em- 
ployment of  more  penetrative  powers,  so  the  first  scintillations 
of  new  natural  phenomena  frequently  present  themselves  to 
the  eye  of  the  observer,  dimly  seen  when  viewed  askance, 
and  disappearing  if  directly  looked  for.  When  new  powers 
of  thought  and  experiment  have  developed  and  corrected  the 
first  notions,  and  given  a  character  to  the  new  image,  proba- 
bly very  different  from  the  first  impression,  fresh  objects  are 


OTHEE   MODES   OF   FORCE. 


177 


again  glanced  at  in  the  margin  of  the  new  field  of  vision, 
which  in  their  turn  have  to  be  verified,  and  again  lead  to 
new  extensions ;  thns  the  effort  to  establish  one  observation 
leads  to  the  imperfect  perception  of  new  and  wider  fields  of 
research;  and,  instead  of  approaching  finality,  the  more 
we  discover,  the  more  infinite  appears  the  range  of  the  undis- 
covered! 

%    8* 


IX.— CONCLUDING  EEMAEKS. 

I  HAVE  now  gone  through  the  affections  of  matter  for 
which  distinct  names  have  been  given  in  our  received 
nomenclature  :  that  other  forces  may  be  detected,  differing  as 
much  from  them  as  they  differ  from  each  other,  is  highly 
probable,  and  that  when  discovered,  and  their  modes  of 
action  fully  traced  out,  they  will  be  found  to  be  related  inter 
se,  and  to  these  forces  as  these  are  to  each  other,  I  believe 
to  be  as  far  certain  as  certainty  can  be  predicted  of  any  future 
event. 

It  may  in  many  cases  be  a  difficult  question  to  determine 
what  constitutes  a  distinct  affection  of  matter  or  mode  of 
force.  It  is  highly  probable  that  different  lines  of  demarca- 
tion would  have  been  drawn  between  the  forces  already 
known,  had  they  been  discovered  in  a  different  manner,  or 
first  observed  at  different  points  of  the  chain  which  connects 
them.  Thus,  radiant  heat  and  light  are  mainly  distinguished 
by  the  manner  in  which  they  affect  our  senses :  were  they 
viewed  according  to  the  way  in  which  they  affect' inorganic 
matter,  very  different  notions  would  possibly  be  entertained 
of  their  character  and  relation.  Electricity,  again,  was 
named  from  the  substance  in  which,  and  magnetism  from  the 
district  where,  it  first  happened  to  be  observed,  and  a  chain 
of  intermediate  phenomena  have  so  connected  electricity  with 
galvanism  that  they  are  now  regarded  as  the  same  force, 


CONCLUDING   EEMAKKS.  179 

differing  only  in  the  degree  of  its  intensity  and  quantity, 
though  for  a  long  time  they  were  regarded  as  distinct. 

The  phenomenon  of  attraction  and  repulsion  by  amber, 
which  originated  the  term  electricity,  is  as  unlike  that  of  the 
decomposition  of  water  by  the  voltaic  pile,  as  any  two  natural 
phenomena  can  well  be.  It  is  only  because  the  historical 
sequence  of  scientific  discoveries  has  associated  them  by  a 
number  of  intermediate  links,  that  they  are  classed  under  the 
same  category.  What  is  called  voltaic  electricity  might 
equally,  perhaps  more  appropriately,  be  called  voltaic  chemis- 
try. I  mention  these  facts  to  show  that  the  distinction  in  the 
name  may  frequently  be  much  greater  than  the  distinction  of 
the  subject  which  it  represents,  and  vice  versa,  not  as  at  all 
objecting  to  the  received  nomenclature  on  these  points ;  nor 
do  I  say  it  would  be  advisable  to  depart  from  it :  were  we  to 
do  so,  inevitable  confusion  would  result,  and  objections 
equally  forcible  might  be  found  to  apply  to  our  new  termi- 
nology. 

Words,  when  established  to  a  certain  point,  become  a 
part  of  the  social  mind ;  its  powers  and  very  existence  de- 
pend upon  the  adoption  of  conventional  symbols ;  and  were 
these  suddenly  departed  from,  or  varied,  according  to  indivi- 
dual apprehensions,  the  acquisition  and  transmission  of  knowl- 
edge would  cease.  Undoubtedly,  neology  is  more  permissi- 
ble in  physical  science  than  in  any  other  branch  of  knowledge, 
because  it  is  more  progressive ;  new  facts  or  new  relations 
require  new  names,  but  even  here  it  should  be  used  with  great 
caution. 

Si  forte  necesse  est 

Indiciis  monstrare  recentibus  abdita  rerum, 
Fingere  cinctutis  non  exaudita  Oethegis, 
Continget ;  dabiturque  licentia,  sumpta  pudenter. 

Even  should  the  mind  ever  be  led  to  dismiss  the  idea  of 
various  forces,  and  regard  them  as  the  exertion  of  one  force, 


180  COBKELATION   OF  PHYSICAL  FORCES. 

or  resolve  them  definitely  into  motion ;  still  we  could  never 
avoid  the  use  of  different  conventional  terms  for  the  different 
modes  of  action  of  this  one  pervading  force. 

Reviewing  the  series  of  relations  between  the  various 
forces  which  we  have  been  considering,  it  would  appear  that 
in  many  cases  where  one  of  these  is  excited  or  exists,  all  the 
others  are  also  set  in  action :  thus,  when  a  substance,  such  as 
sulphuret  of  antimony,  is  electrified,  at  the  instant  of  electri- 
sation it  becomes  magnetic  in  directions  at  right  angles  to  the 
lines  of  electric  force  ;  at  the  same  time  it  becomes  heated  to 
an  extent  greater  or  less  according  to  the  intensity  of  the 
electric  force.  If  this  intensity  be  exalted  to  a  certain  point 
the  sulphuret  becomes  luminous,  or  light  is  produced :  it  ex- 
pands, consequently  motion  is  produced ;  and  it  is  decomposed, 
therefore  chemical  action  is  produced.  If  we  take  another 
substance,  say  a  metal,  all  these  forces  except  the  last  are 
developed ;  and  although  we  can  scarcely  apply  the  term 
mechanical  action  to  a  substance  hitherto  undecomposed,  and 
which,  under  the  circumstances  we  are  considering,  enters 
into  no  new  combination,  yet  it  undergoes  that  species  of 
polarisation  which,  as  far  as  we  can  judge,  is  the  first  step 
towards  chemical  action,  and  which,  if  the  substance  were 
decomposable,  would  resolve  it  into  its  elements.  Perhaps, 
indeed,  some  hitherto  undiscovered  chemical  action  is  pro- 
duced in  substances  which  we  regard  as  undecomposable : 
there  are  experiments  to  show  that  metals  which  have  been 
electrised  are  permanently  changed  in  their  molecular  consti- 
tution. Oxygen,  we  have  seen,  is  changed  by  the  electric 
spark  into  ozone,  and  phosphorus  into  allotropic  phosphorus, 
both  which  changes  were  for  a  long  time  unknown  to  those 
familiar  with  electrical  science. 

Thus,  with  some  substances,  when  one  mode  of  force  is 
produced  all  the  others  are  simultaneously  developed.  With 
other  substances,  probably  with  all  matter,  some  of  the  other 
forces  are  developed,  whenever  one  is  excited,  and  all  may  be 


CONCLUDING   REMARKS.  181 

so  were  the  matter  in  a  suitable  condition  for  their  develope- 
ment,  or  our  means  of  detecting  them  sufficiently  delicate. 

This  simultaneous  production  of  several  different  forces 
seems  at  first  sight  to  be  irreconcileable  with  their  mutual  and 
necessary  dependence,  and  it  certainly  presents  a  formidable 
experimental  difficulty  in  the  way  of  establishing  their  equiv- 
alent relations  ;  but  when  examined  closely,  it  is  not  in  fact 
inconsistent  with  the  views  we  have  been  considering,  but  is 
indeed  a  strong  argument  in  favour  of  the  theory  which  re- 
gards them  as  modes  of  motion. 

Let  us  select  one  or  two  cases  in  which  this  form  of  ob- 
jection may  be  prominently  put  forward.  A  voltaic  battery 
decomposing  water  in  a  voltameter,  while  the  same  current 
is  employed  at  the  same  time  to  make  an  electro-magnet, 
gives  nevertheless  in  the  voltameter  an  equivalent  of  gas,  or 
decomposes  an  equivalent  of  an  electrolyte  for  each  equiva- 
lent of  chemical  decomposition  in  the  battery  cells,  and  will 
give  the  same  ratios  if  the  electro-magnet  be  removed.  Here, 
at  first  sight,  it  would  appear  that  the  magnetism  was  an  ex- 
tra force  produced,  and  that  thus  more  than  the  equivalent 
power  was  obtained  from  the  battery.  In  answer  to  this 
objection  it  may  be  said,  that  in  the  circumstances  under 
which  this  experiment  is  ordinarily  performed,  several  cells 
of  the  battery  are  used,  and  so  there  is  a  far  greater  amount 
of  force  generated  in  the  cells  than  is  indicated  by  the  effect 
in  the  voltameter.  If,  moreover,  the  magnet  be  not  inter- 
posed, still  the  magnetic  force  is  equally  existent  throughout 
the  whole  current ;  for  instance,  the  wires  joining  the  plates 
will  attract  iron  filings,  deflect  magnetic  needles,  &c.,  and 
produce  diamagnetic  effects  on  surrounding  matter.  By  the 
iron  core  a  small  portion  of  the  force  is,  indeed,  absorbed 
while  it  is  being  made  a  magnet,  but  this  ceases  to  be  ab- 
sorbed when  the  magnet  is  made  ;  this  has  been  proved  by 
the  observation  of  Mr.  Latimer  Clarke,  who  has  found  that 
along  the  wires  of  the  electric  telegraph  the  magnetic  needles 


182  CORRELATION   OF   PHYSICAL    FORCES. 

placed  at  different  stations  remained  fixed  after  the  connection 
with  the  battery  was  made,  and  while  the  electric  current 
acted  by  induction  on  surrounding  conducting  matter,  separa- 
ted from  the  wires  by  their  gutta  percha  coating,  so  that  a 
sort  of  Leyden  phial  was  formed  ;  but  as  soon  as  this  induc- 
tion had  produced  its  effect  between  each  station,  or,  so  to 
speak,  the  phial  was  charged,  the  needles  successively  were 
deflected :  it  is  like  the  case  of  a  pulley  and  weight,  which  lat- 
ter exhausts  force  while  it  is  being  raised ;  but  when  raised, 
the  force  is  free,  and  may  be  used  for  other  purposes. 

If  a  battery  of  one  cell,  just  capable  of  decomposing  water 
and  no  more,  be  employed,  this  will  cease  to  decompose  while 
making  a  magnet.  There  must,  in  every  case,  be  prepon- 
derating chemical  affinity  in  the  battery  cells,  either  by  the 
nature  of  its  elements  or  by  the  reduplication  of  series,  to 
effect  decomposition  in  the  voltameter ;  and  if  the  point  is 
just  reached  at  which  this  is  effected,  and  the  power  is  then 
reduced  by  any  resistance,  decomposition  ceases :  were  it 
otherwise,  were  the  decomposition  in  the  voltameter  the 
exponent  of  the  entire  force  of  the  generating  cells,  and  these 
could  independently  produce  magnetic  force,  this  latter 
force  would  be  got  from  nothing,  and  perpetual  motion  be 
obtained. 

To  take  another  and  different  example :  A  piece  of  zinc 
dissolved  in  dilute  sulphuric  acid  gives  somewhat  less  heat  than 
when  the  zinc  has  a  wire  of  platinum  attached  to  it,  and  is 
dissolved  by  the  same  quantity  of  acid.  The  argument  is 
deduced  that,  as  there  is  more  electricity  in  the  second  than 
in  the  first  case,  there  should  be  less  heat ;  but  as,  according 
to  our  received  theories,  the  heat  is  a  product  of  the  electric 
current,  and  in  consequence  of  the  impurity  of  zinc  electrici- 
ty is  generated  in  the  first  case  molecularly,  in  what  is  called 
local  action,  though  not  thrown  into  a  general  direction, 
there  should  be  more  of  both  heat  and  electricity  in  the  sec- 
ond than  in  the  first  case,  as  the  heat  and  electricity  due  to 


CONCLUDING   REMARKS.  183 

the  voltaic  combination  of  zinc  and  platinum  are  added  to 
that  excited  on  the  surface  of  the  zinc,  and  the  zinc  should  be, 
as  in  fact  it  is,  more  rapidly  dissolved ;  so  that  the  extra  heat 
and  electricity  is  produced  by  extra  chemical  force.  Many 
additional  cases  of  a  similar  description  might  be  suggested. 
But  although  it  is  difficult,  an4  perhaps  impossible,  to  restrict 
the  action  of  any  one  force  to  the  production  of  one  other 
force,  and  of  one  only — yet  if  the  whole  of  one  force,  say 
chemical  action,  be  supposed  to  be  employed  in  producing  its 
full  equivalent  of  another  force,  say  heat,  then  as  this  heat  is 
capable  in  its  turn  of  reproducing  chemical  action,  and  in  the 
limit,  a  quantity  equal  or  at  least  only  infinitely  short  of  the 
initial  force  :  if  this  could  at  the  same  time  produce  indepen- 
dently another  force,  say  magnetism,  we  could,  by  adding 
the  magnetism  to  the  total  heat,  get  more  than  the  original 
chemical  action,  and  thus  create  force  or  obtain  perpetual 
motion. 

The  term  Correlation,  which  I  selected  as  the  title  of  my 
Lectures  in  1843,  strictly  interpreted,  means  a  necessary 
mutual  or  reciprocal  dependence  of  two  ideas,  inseparable 
even  in  mental  conception :  thus,  the  idea  of  height  cannot 
exist  without  involving  the  idea  of  its  correlate,  depth ;  the 
idea  of  parent  cannot  exist  without  involving  the  idea  of  off- 
spring. It  has  been  scarcely,  if  at  all,  used  by  writers  on 
physics,  but  there  are  a  vast  variety  of  physical  relations  to 
which,  if  it  does  not  in  its  strictest  original  sense  apply, 
cannot  certainly  be  so  well  expressed  by  any  other  term. 
There  are,  for  example,  many  facts,  one  of  which  cannot  take 
place  without  involving  the  other ;  one  arm  of  a  lever  can- 
not be  depressed  without  the  other  being  elevated — the  finger 
cannot  press  the  table  without  the  table  pressing  the  finger. 
A  body  cannot  be  heated  without  another  being  cooled,  or 
some  other  force  being  exhausted  in  an  equivalent  ratio  to 
the  production  of  heat ;  a  body  cannot  be  positively  elec- 
trified without  some  other  body  being  negatively  electri- 
fied, &c. 


184  CORRELATION  OF  PHYSICAL   FORCES. 

The  probability  is,  that,  if  not  all,  the  greater  number  of 
physical  phenomena  are  correlative,  and  that,  without  a 
duality  of  conception,  the  mind  cannot  form  an  idea  of  them : 
thus  motion  cannot  be  perceived  or  probably  imagined  with- 
out parallax  or  relative  change  of  position.  The  world  was 
believed  fixed,  until  by  comparison  with  the  celestial  bodies, 
it  was  found  to  change  its  place  with  regard  to  them :  had 
there  been  no  perceptible  matter  external  to  the  world,  we 
should  never  have  discovered  its  motion.  In  sailing  along  a 
river,  the  stationary  vessels  and  objects  on  the  banks  seem 
to  move  past  the  observer  :  if  at  last  he  arrives  at  the  convic- 
tion that  he  is  moving,  and  not  these  objects,  it  is  by  correct- 
ing his  senses  by  reflection  derived  from  a  more  extensive 
previous  use  of  them :  even  then  he  can  only  form  a  notion 
of  the  motion  of  the  vessel  he  is  in,  by  its  change  of  position 
with  regard  to  the  objects  it  passes — that  is,  provided  his 
body  partakes  of  the  motion  of  the  vessel,  which  it  only  does 
when  its  course  is  perfectly  smooth,  otherwise  the.  relative 
change  of  position  of  the  different  parts  of  the  body  and  the 
vessel  inform  him  of  its  alternating,  though  not  of  its  pro- 
gressive movement.  So  in  all  physical  phenomena,  the  effects 
produced  by  motion  are  all  in  proportion  to  the  relative  mo- 
tion :  thus,  whether  the  rubber  of  an  electrical  machine  be 
stationary,  and  the  cylinder  mobile,  or  the  rubber  mobile  and 
the  cylinder  stationary,  or  both  mobile  in  different  directions, 
or  in  the  same  direction  with  different  degrees  of  velocity, 
the  electrical  effects  are,  cceteris  paribus,  precisely  the  same, 
provided  the  relative  motion  is  the  same,  and  so,  without  ex- 
ception, of  all  other  phenomena.  The  question  of  whether 
there  can  be  absolute  motion,  or,  indeed,  any  absolute  isolated 
force,  is  purely  the  metaphysical  question  of  idealism  or  real- 
ism— a  question  for  our  purpose  of  little  import ;  sufficient 
for  the  purely  physical  inquirer,  the  maxim  '  de  non  apparenti- 
bus  et  non  existentibus  eadem  est  ratio.9 

The  sense  I  have  attached  to  the  word  correlation,  in 


CONCLUDING  KEMAEKS.  185 

treating  of  physical  phenomena,  will,  I  think,  be  evident  from 
the  previous  parts  of  this  essay,  to  be  that  of  a  necessary 
reciprocal  production  :  in  other  words,  that  any  force  capable 
of  producing  another  may,  in  its  turn,  be  produced  by  it — 
nay,  more,  can  be  itself  resisted  by  the  force  it  produces,  in 
proportion  to  the  energy  of  such  production,  as  action  is  ever 
accompanied  and  resisted  by  reaction  :  thus,  the  action  of  an 
electro-magnetic  machine  is  reacted  upon  by  the  magneto- 
electricity  developed  by  its  action. 

To  many,  however,  of  the  cases  we  have  been  consider- 
ing, the  term  correlation  may  be  applied  in  a  more  strict 
accordance  with  its  original  sense :  thus,  with  regard  to  the 
forces  of  electricity  and  magnetism  in  a  dynamic  state,  we 
cannot  electrise  a  substance  without  magnetising  it — we  can- 
not magnetise  it  without  electrising  it : — each  molecule,  the 
instant  it  is  affected  by  one  of  these  forces,  is  affected  by  the 
other ;  but,  in  transverse  directions,  the  forces  are  insepara- 
ble and  mutually  dependent — correlative,  but  not  identical. 

The  evolution  of  one  force  or  mode  of  force  into  another 
has  induced  many  to  regard  all  the  different  natural  agencies 
as  reducible  to  unity,  and  as  resulting  from  one  force  which 
is  the  efficient  cause  of  all  the  others  :  thus,  one  author  writes 
to  prove  that  electricity  is  the  cause  of  every  change  in 
matter ;  another,  that  chemical  action  is  the  cause  of  every- 
thing ;  another,  that  heat  is  the  universal  cause,  and  so  on. 
If,  as  I  have  stated  it,  the  true  expression  of  the  fact  is,  that 
each  mode  of  force  is  capable  of  producing  the  others,  and 
that  none  of  them  can  be  produced  but  by  some  other  as  an 
anterior  force,  then  any  view  which  regards  either  of  them  as 
abstractedly  the  efficient  cause  of  all  the  rest,  is  erroneous ; 
the  view  has,  I  believe,  arisen  from  a  confusion  between  the 
abstract  or  generalised  meaning  of  the  term  cause,  and  its 
concrete  or  special  sense ;  the  word  itself  being  indiscrimi- 
nately used  in  both  these  senses. 

Another  confusion  of  terms  has  arisen,  and  has,  indeed, 


186  CORRELATION    OF   PHYSICAL   FORCES. 

much  embarrassed  me  in  enunciating  the  propositions  put 
forth  in  these  pages,  on  account  of  the  imperfection  of  scien- 
tific language  ;  an  imperfection  in  great  measure  unavoidable, 
it  is  true,  but  not  the  less  embarrassing.  Thus,  the  words 
light,  heat,  electricity,  and  magnetism,  are  constantly  used  in 
two  senses — viz.  that  of  the  force  producing,  or  the  subject- 
ive idea  of  force  or  power,  and  of  the  effect  produced,  or  the 
objective  phenomenon.  The  word  motion,  indeed,  is  only 
applied  to  the  effect,  and  not  to  the  force,  and  the  term  chem- 
ical affinity  is  generally  applied  to  the  force,  and  not  to  the 
effect ;  but  the  other  four  terms  are,  for  want  of  a  distinct 
terminology,  applied  indiscriminately  to  both. 

I  may  have  occasionally  used  the  same  word  at  one  time 
in  a  subjective,  at  another  in  an  objective  sense ;  all  I  can 
say  is,  that  this  cannot  be  avoided  without  a  neology,  which 
I  have  not  the  presumption  to  introduce,  or  the  authority  to 
enforce.  Again,  the  use  of  the  term  forces  in  the  plural 
might  be  objected  to  by  those  who  do  not  attach  to  the  term 
force  the  notion  of  a  specific  agency,  but  of  one  universal 
power  associated  with  matter,  of  which  its  various  phenom- 
ena are  but  diversely  modified  effects. 

Whether  the  imponderable  agents,  viewed  as  force,  and 
not  as  matter,  ought  to  be  regarded  as  distinct  forces  or  as 
distinct  modes  of  force,  is  probably  not  very  material,  for,  as 
far  as  I  am  aware,  the  same  result  would  follow  either  view ; 
I  have  therefore  used  the  terms  indiscriminately,  as  either 
happened  to  be  the  more  expressive  for  the  occasion. 

Throughout  this  essay  I  have  placed  motion  in  the  same 
category  as  the  other  affections  of  matter.  The  course  of 
reasoning  adopted  in  it,  however,  appears  to  me  to  lead  inev- 
itably to  the  conclusion  that  these  affections  of  matter  are 
themselves  modes  of  motion ;  that,  as  in  the  case  of  friction, 
the  gross  or  palpable  motion,  which  is  arrested  by  the  con- 
tact of  another  body,  is  subdivided  into  molecular  motions  or 
vibrations,  which  vibrations  are  heat  or  electricity,  as  the 


CONCLUDING  REMABK6.  187 

case  may  be  ;  so  the  other  affections  are  only  matter  moved 
or  molecularly  agitated  in  certain  definite  directions.  "We 
have  already  considered  the  hypothesis  that  the  passage  of 
electricity  and  magnetism  causes  vibrations  in  an  ether  per- 
meating the  bodies  through  which  the  current  is  transmitted, 
or  the  application  of  the  same  ethereal  hypothesis  to  these 
imponderables  which  had  previously  been  applied  to  light ; 
many,  in  speaking  of  some  of  the  effects,  admit  that  electri- 
city and  magnetism  cause  or  produce  by  their  passage  vibra- 
tions in  the  particles  of  matter,  but  regard  the  vibrations 
produced  as  an  occasional,  though  not  always  a  necessary, 
effect  of  the  passage  of  electricity,  or  of  the  increment  or 
decrement  of  magnetism.  The  view  which  I  have  taken  is, 
that  such  vibrations,  molecular  polarisations,  or  motions  of 
some  sort  from  particle  to  particle,  are  themselves  electricity 
or  magnetism ;  or,  to  express  it  in  the  converse,  that  dynamic 
electricity  and  magnetism  are  themselves  motion,  and  that 
permanent  magnetism,  and  Franklinic  electricity,  are  static 
conditions  of  force  bearing  a  similar  relation  to  motion  which 
tension  or  gravitation  do. 

This  theory  might  well  be  discussed  in  greater  detail 
than  has  been  used  in  this  work ;  but  to  do  this  and  to  anti- 
cipate objections  would  lead  into  specialities  foreign  to  my 
present  object,  in  the  course  of  this  essay  my  principal  aim 
having  been  rather  to  show  the  relation  of  forces  as  evinced 
by  acknowledged  facts,  than  to  enter  upon  any  detailed  ex- 
planation of  their  specific  modes  of  action. 

Probably  man  will  never  know  the  ultimate  structure  of 
matter  or  the  minutise  of  molecular  actions ;  indeed  it  is 
scarcely  conceivable  that  the  mind  can  ever  attain  to  this 
knowledge ;  the  monad  irresolvable  by  a  given  microscope 
may  be  resolved  by  an  increase  in  power.  Much  harm  has 
already  been  done  by  attempting  hypothetically  to  dissect 
matter  and  to  discuss  the  shapes,  sizes,  and  numbers  of  at- 
oms, and  their  atmospheres  of  heat,  ether,  or  electricity. 


188  COKEELATION   OF  PHYSICAL   FOBOES. 

Whether  the  regarding  electricity,  light,  magnetism,  &c., 
as  simply  motions  of  ordinary  matter,  be  or  be  not  admissi- 
ble, certain  it  is,  that  all  past  theories  have  resolved,  and  all 
existing  theories  do  resolve,  the  actions  of  these  forces  into 
motion.  Whether  it  be  that,  on  account  of  our  familiarity 
with  motion,  we  refer  other  affections  to  it,  as  to  a  language 
which  is  most  easily  construed  and  most  capable  of  explain- 
ing them ;  whether  it  be  that  it  is  in  reality  the  only  mode  in 
which  our  minds,  as  contradistinguished  from  our  senses,  are 
able  to  conceive  material  agencies ;  certain  it  is,  that  since 
the  period  at  which  the  mystic  notions  of  spiritual  or  preter- 
natural powers  were  applied  to  account  for  physical  phenom- 
ena, all  hypotheses  framed  to  explain  them  have  resolved 
them  into  motion.  Take,  for  example,  the  theories  of  light 
to  which  I  have  before  alluded :  one  of  these  supposes  light 
to  be  a  highly  rare  matter,  emitted  from — i.  e.  put  in  motion 
by — luminous  bodies ;  a  second  supposes  that  the  matter  is 
not  emitted  from  luminous  bodies,  but  that  it  is  put  into  a 
state  of  vibration  or  undulation,  i.  e.  motion,  by  them ;  and 
thirdly,  light  may  be  regarded  as  an  undulation  or  motion  of 
ordinary  matter,  and  propagated  by  undulation  of  air,  glass, 
&c.,  as  I  have  before  stated.  In  all  these  hypotheses,  matter 
and  motion  are  the  only  conceptions.  Nor,  if  we  accept 
terms  derived  from  our  own  sensations,  the  which  sensations 
themselves  may  be  but  modes  of  motion  in  the  nervous  fila- 
ments, can  we  find  words  to  describe  phenomena  other  than 
those  expressive  of  matter  and  motion.  We  in  vain  struggle 
to  escape  from  these  ideas ;  if  we  ever  do  so,  our  mental 
powers  must  undergo  a  change  of  which  at  present  we  see 
no  prospect. 

If  we  apply  to  any  other  force  the  mode  of  reasoning 
which  we  have  applied  to  heat,  we  shall  arrive  at  the  same 
conclusion,  and  see  that  a  given  source  of  power  can,  sup- 
posing it  to  be  fully  utilised  in  each  case,  yield  no  more  by 
employing  it  as  an  exciter  of  one  force  than  of  another.  Let 


CONCLUDING  REMARKS.  189 

us  take  electricity  as  an  example.  Suppose  a  pound  of  mer- 
cury at  400°  be  employed  to  produce  a  thermo-electric  cur- 
rent, and  the  latter  be  in  its  turn  employed  to  produce  me- 
chanical force  ;  if  this  latter  force  be  greater  than  that  which 
the  direct  effect  of  heat  would  produce,  then  it  could  by  com- 
pression raise  the  temperature  of  the  mercury  itself,  or  of  a 
similar  quantity  equally  heated,  to  a  higher  point  than  its 
original  temperature,  the  400°  to  401°,  for  example,  which 
is  obviously  Impossible ;  nor,  if  we  admit  force  to  be  inde- 
structible, can  it  produce  less  than  400°,  or  cool  the  second 
body  except  by  some  portion  of  it  being  converted  into 
another  form  or  mode  of  force. 

But  as  the  mechanical  effect  here  is  produced  through  the 
medium  of  electricity,  and  the  mechanical  effect  is  definite, 
so  the  quantity  of  electricity  producing  it  must  be  definite 
also,  for  unequal  quantities  of  electricity  could  only  produce 
an  equal  mechanical  effect  by  a  loss  or  gain  of  their  own 
force  into  or  out  of  nothing.  The  same  reasoning  will  apply 
to  the  other  forces,  and  will  lead,  it  appears  to  me,  necessa- 
rily and  inevitably  to  the  conclusion,  that  each  force  is  defi- 
nitely and  equivalently  convertible  into  any  other,  and  that 
where  experiment  does  not  give  the  full  equivalent,  it  is  be- 
cause the  initial  force  has  been  dissipated,  not  lost,  by  con- 
version into  other  unrecognised  forces.  The  equivalent  is  the 
limit  never  practically  reached. 

The  great  problem  which  remains  to  be  solved,  in  regard 
to  the  correlation  of  physical  forces,  is  this  establishment  of 
their  equivalents  of  power,  or  their  measurable  relation  to  a 
given  standard.  The  progress  made  in  some  of  the  branches 
of  this  inquiry  has  been  already  noticed.  Viewed  in  their 
static  relations,  or  in  the  conditions  requisite  for  producing 
equilibrium  or  quantitative  equality  of  force,  a  remarkable 
relation  between  chemical  affinity  and  heat  is  that  discovered 
in  many  simple  bodies  by  Dulong  and  Petit,  and  extended  to 
compounds  by  Neumann  and  Avogadro.  Their  researches 


190  CORBEL ATION   OF  PHYSICAL  FORCES. 

have  shown  that  the  specific  heats  of  certain  substances, 
when  multiplied  by  their  chemical  equivalents,  give  a  con- 
stant quantity  as  product — or,  in  other  words,  that  the  com- 
bining weights  of  such  substances  are  those  weights  which 
require  equal  accessions  or  abstractions  of  heat,  equally  to 
raise  or  lower  their  temperature.  To  put  the  proposition 
more  in  accordance  with  the  view  we  have  taken  of  the  na- 
ture of  heat :  each  body  has  a  power  of  communicating  or 
receiving  molecular  repulsive  power,  exactly  equal,  weight 
for  weight,  to  its  chemical  or  combining  power.  For  in- 
stance, the  equivalent  of  lead  is  104,  of  zinc  33,  or,  in  round 
numbers,  as  3  to  1 :  these  numbers  are  therefore  inversely 
the  exponents  of  their  chemical  power,  three  times  as  much 
lead  as  zinc  being  required  to  saturate  the  same  quan- 
tity of  an  acid  or  substance  combining  with  it ;  but  their 
power  of  communicating  or  abstracting  heat  or  repulsive 
power  is  precisely  the  same,  for  three  times  as  much  lead  as 
zinc  is  required  to  produce  the  same  amount  of  expansion  or 
contraction  in  a  given  quantity  of  a  third  substance,  such  as 
water. 

Again,  a  great  number  of  bodies  chemically  combine  in 
equal  volumes,  i.  e.  in  the  ratios  of  their  specific  gravities ; 
but  the  specific  gravities  represent  the  attractive  powers  of 
the  substance,  or  are  the  numerical  exponents  of  the  forces 
tending  to  produce  motion  in  masses  of  matter  towards  each 
other ;  while  the  chemical  equivalents  are  the  exponents  of 
the  affinities  or  tendencies  of  the  molecules  of  dissimilar  sub- 
stances to  combine,  and  saturate  each  other ;  consequently, 
here  we  have  to  some  extent  an  equivalent  relation  between 
these  two  modes  of  force— gravitation  and  chemical  attrac- 
tion. 

Were  the  above  relations  extended  into  an  universal  law, 
we  should  have  the  same  numerical  expression  for  the  three 
forces  of  heat,  gravity,  and  affinity ;  and  as  electricity  and 
magnetism  are  quantitatively  related  to  them,  we  should  have 


CONCLUDING  REMARKS.^*  191 

a  similar  expression  for  these  forces^:  but  at  present  the  bod- 
ies in  which  this  parity  of  force  has  been  discovered,  though 
in  themselves  numerous,  are  small  compared  with  the  excep- 
tions, and,  therefore,  this  point  can  only  be  indicated  as  prom- 
ising a  generalisation,  should  subsequent  researches  alter  our 
knowledge  as  to  the  elements  and  combining  equivalents  of 
matter. 

"With  regard  to  what  may  be  called  dynamic  equivalents, 
i.  e.  the  definite  relation  to  time  of  the  action  of  these  varied 
forces  upon  equivalents  of  matter,  the  difficulty  of  establish- 
ing them  is  still  greater.  If  the  proposition  which  I  stated 
at  the  commencement  of  this  paper  be  correct,  that  motion 
may  be  subdivided  or  changed  in  character,  so  as  to  become 
heat,  electricity,  &c.,  it  ought  to  follow  that  when  we  collect 
the  dissipated  and  changed  forces,  and  reconvert  them,  the 
initial  motion,  minus  an  infinitesimal  quantity  affecting  the 
same  amount  of  matter  with  the  same  velocity,  should  be  re- 
produced, and  so  of  the  changes  in  matter  produced  by  the 
other  forces  ;  but  the  difficulties  of  proving  the  truth  of  this 
by  experiment  will,  in  many  cases,  be  all  but  insuperable ; 
we  cannot  imprison  motion  as  we  can  matter,  though  we  may 
to  some  extent  restrain  its  direction. 

The  term  perpetual  motion,  which  I  have  not  unfrequent- 
ly  employed  in  these  pages,  is  itself  equivocal.  If  the  doc- 
trines here  advanced  be  founded,  all  motion  is,  in  one  sense, 
perpetual.  In  masses  whose  motion  is  stopped  by  mutual 
concussion,  heat  or  motion  of  the  particles  is  generated ;  and 
thus  the  motion  continues,  so  that  if  we  could  venture  to  extend 
such  thoughts  to  the  universe,  we  should  assume  the  same 
amount  of  motion  affecting  the  same  amount  of  matter  forever. 
Where  force  opposes  force,  as  in  cases  of  static  equilibrium, 
the  balance  of  pre-existing  equilibrium  is  affected,  and  fresh 
motion  is  started  equivalent  to  that  which  is  withdrawn  into 
a  state  of  abeyance. 

But  the  term  perpetual  motion  is  applied,  in  ordinary  par- 


192  COEEELATION   OF   PHYSICAL   FOECES. 

lance  (and  in  such  sense  1  have  used  it),  to  a  perpetual  recur- 
rent motion,  e.g.  a  weight  which  by  its  fall  would  turn  a 
wheel,  which  wheel  would,  in  its  turn,  raise  the  initial  weight, 
and  so  on  forever,  or  until  the  material  of  which  the  machine 
is  made  be  worn  out.  It  is  strange  that  to  common  appre- 
hension the  impossibility  of  this  is  not  self-evident :  if  the  in- 
itial weight  is  to  be  raised  by  the  force  it  has  itself  generated, 
it  must  necessarily  generate  a  force  greater  than  that  of  its 
own  weight  or  centripetal  attraction  ;  in  other  words,  it  must 
be  capable  of  raising  a  weight  heavier  than  itself :  so  that, 
setting  aside  the  resistance  of  friction,  &c.,  a  weight,  to  pro- 
duce perpetual  recurrent  motion,  must  be  heavier  than  an 
equal  weight  of  matter,  in  short,  heavier  than  itself. 

"  Suppose  two  equal  weights  at  each  end  of  an  equi-armed 
lever,  there  is  no  motion ;  cut  off  a  fraction  of  one  of  them, 
and  it  rises  while  the  other  falls.  How,  now,  is  the  lesser 
weight  to  bring  back  the  greater  without  any  extraneous  ap- 
plication of  force  ?  If,  as  is  obvious,  it  cannot  do  so  in  this 
simple  form  of  experiment,  it  is  a  fortiori  more  impossible  if 
machinery  be  added,  for  increased  resistances  have  then  to 
be  overcome.  Can  we  again  mend  this  by  employing  any 
other  force?  Suppose  we  employ  electricity,  the  initial 
weight  in  descending  turns  a  cylinder  against  a  cushion,  and 
so  generates  electricity ;  to  make  this  force  recurrent,  the 
electricity  so  generated  must,  in  its  turn,  raise  the  initial 
weight,  or  one  heavier  than  it,  i.  e.  the  initial  weight  must, 
through  the  medium  of  electricity,  raise  a  weight  heavier  than 
itself.  The  same  problem,  applied  to  any  other  forces,  will 
involve  the  same  absurdity :  and  yet  simple  as  the  matter 
seems,  the  world  is  hardly  yet  disabused  of  an  idea  little  re- 
moved from  superstition. 

But  the  importance  of  the  deductions  to  be  derived  from 
the  negation  of  perpetual  motion  seems  scarcely  to  have  im- 
pressed philosophers,  and  we  only  find  here  and  there  a  scat- 
tered hint  of  the  consequences  necessarily  resulting  from  that 


CONCLUDING  REMARKS.  193 

which  to  the  thinking  mind  is  a  conviction.  Some  of  these 
I  have  ventured  to  put  forward  in  the  present  essay,  but 
many  remain,  and  will  crowd  upon  the  mind  of  those  who 
pursue  the  subject.  Does  not,  for  instance,  the  impossibility 
of  perpetual  motion,  when  thought  out,  involve  the  demon- 
stration of  the  impossibility,  to  which  I  have  previously  allud- 
ed, of  any  event  identically  recurring  ? 

The  pendulum  in  vacuo,  at  each  beat  leaves  a  portion  of 
the  force  which  started  it  in  the  form  of  heat  at  its  point  of 
suspension :  this  force,  though  ever  existent,  can  never  be  re- 
stored in  its  integrity  to  the  ball  of  the  pendulum,  for  in  the 
process  of  restoration  it  must  affect  other  matter,  and  alter 
the  condition  of  the  universe.  To  restore  the  initial  force  to  its 
integrity,  everything  as  it  existed  at  the  moment  of  the  first 
beat  of  the  pendulum  must  be  restored  in  its  integrity :  but 
how  can  this  be — for  while  the  force  was  escaping  from  the 
pendulum  by  radiating  heat  from  the  point  of  suspension, 
surrounding  matter  has  not  stood  still ;  the  very  attraction 
which  caused  the  beat  of  the  pendulum  has  changed  in  degree, 
for  the  pendulum  is  nearer  to  or  further  from  the  sun,  or 
from  some  planet  or  fixed  star. 

It  might  be  an  interesting  and  not  profitless  speculation 
to  follow  out  these  and  other  consequences  ;  it  would,  I  be- 
lieve, lead  us  to  the  conviction  that  the  universe  is  ever 
changing,  and  that  notwithstanding  secular  recurrences  which 
would  prima  facie  seem  to  replace  matter  in  its  original  posi- 
tion, nothing  in  fact  ever  returns  or  can  return  to  a  state  of 
existence  identical  with  a  previous  state.  But  the  field  is  too 
illimitable  for  me  to  venture  further. 

The  inevitable  dissipation  or  throwing  off  a  portion  of 
the  initial  force  presents  a  great  experimental  difficulty  in  the 
way  of  establishing  the  equivalents  of  the  various  natural 
forces.  In  the  steam-engine,  for  instance,  the  heat  of  the 
furnace  not  only  expands  the  water  and  thereby  produces  the 
motion  of  the  piston,  but  it  also  expands  the  iron  of  the  boil- 
9 


194  CORRELATION   OF  PHYSICAL  FORCES. 

er,  of  the  cylinder  and  all  surrounding  bodies.  The  force  ex- 
pended in  expanding  this  iron  to  a  very  small  extent  is  equal 
to  that  which  expands  the  vapour  to  a  very  large  extent :  this 
expansion  of  the  iron  is  capable,  in  its  turn,  of  producing  a 
great  mechanical  force,  which  is  practically  lost.  Could  all 
the  force  be  applied  to  the  vapour,  an  enormous  addition  of 
power  would  be  gained  for  the  same  expenditure  :  and  per- 
haps even  with  our  present  means  more  might  be  done  in 
utilising  the  expansion  of  the  iron. 

Another  great  difficulty  in  experimentally  ascertaining  the 
dynamic  equivalents  of  different  forces  arises  from  the  effects 
of  disruption,  or  the  overcoming  an  existing  force.  Thus, 
when  a  part  of  the  initial  force  employed  is  engaged  in  twist- 
ing or  tearing  asunder  matter  previously  held  together  by 
cohesive  attraction,  or  in  overcoming  gravitation  or  inertia, 
the  same  amount  of  heat  or  electricity  would  not  be  evolved 
as  if  such  obstacle  were  non-existent,  and  the  initial  force 
were  wholly  employed  in  producing,  not  in  opposing.  There 
is  a  difficulty  apparently  extreme  in  devising  experiments 
in  which  some  portion  of  the  force  is  not  so  employed. 

The  initial  force,  however,  that  has  been  employed  for  such 
disruption  is  not  lost,  as  at  the  moment  of  disruption  the 
bodies  producing  it  fly  off,  and  carry  with  them  their  force. 
Thus,  let  two  weights  be  attached  to  a  cord  placed  across  a 
bar ;  when  their  force  is  sufficient  to  break  the  cord  or  the 
bar,  the  weights  fall  down  and  strike  the  earth,  making  it 
vibrate,  and  so  conveying  away  or  continuing  the  force  ex- 
pressed by  the  cohesion  of  the  bar  or  cord.  If,  instead  of 
breaking  a  cord,  the  weights  be  employed  to  bend  a  bar,  their 
gravitating  force,  instead  of  making  the  earth  vibrate,  pro- 
duces heat  in  the  bar,  and  so  with  whatever  other  force  be 
employed  to  produce  effects  of  disruption,  torsion,  &c.,  so 
that,  though  difficult  in  practice,  the  numerical  problem  of 
the  equivalent  of  the  force  is  not  theoretically  irresolvable. 

The  voltaic  battery  affords  us  the  best  means  of  ascertain- 


CONCLUDING  REMAUKS.  195 

ing  the  dynamic  equivalents  of  different  forces,  and  it  is  probable 
that  by  its  aid  the  best  theoretical  and  practical  re  suits  will  be 
ultimately  attained. 

In  investigating  the  relation  of  the  different  forces,  I  have 
in  .turn  taken  each  one  as  the  initial  force  or  starting-point, 
and  endeavoured  to  show  how  the  force  thus  arbitrarily  se- 
lected could  mediately  or  immediately  produce  and  be  merged 
into  the  others :  but  it  will  be  obvious  to  those  who  have  at- 
tentively considered  the  subject,  and  brought  their  minds  into 
a  general  accordance  with  the  views  I  have  submitted  to  them, 
that  no  force  can,  strictly  speaking,  be  initial,  as  there  must 
be  some  anterior  force  which  produced  it :  we  cannot  create 
force  or  motion  any  more  than  we  can  create  matter. 

Thus,  to  take  an  example  previously  noticed,  and  recede 
backwards ;  the  spark  of  light  is  produced  by  electricity, 
electricity  by  motion,  and  motion  is  produced  by  something 
else,  say  a  steam-engine — that  is,  by  heat.  This  heat  is  pro- 
duced by  chemical  affinity,  i.e.  the  affinity  of  the  carbon  of 
the  coal  for  the  oxygen  of  the  air :  this  carbon  and  this  oxy- 
gen have  been  previously  eliminated  by  actions  difficult  to 
trace,  but  of  the  pre-existence  of  which  we  cannot  doubt, 
and  in  which  actions  we  should  find  the  conjoint  and  al- 
ternating effects  of  heat,  light,  chemical  affinity,  &c.  Thus, 
tracing  any  force  backwards  to  its  antecedents,  we  are  merged 
in  an  infinity  of  changing  forms  of  force ;  at  some  point  we 
lose  it,  not  because  it  has  been  in  fact  created  at  any  definite 
point,  but  because  it  resolves  itself  into  so  many  contributing 
forces,  that  the  evidence  of  it  is  lost  to  our  senses  or  powers 
of  detection ;  just  as  in  following  it  forward  into  the  effect  it 
produces,  it  becomes,  as  I  have  before  stated,  so  subdivided 
and  dissipated  as  to  be  equally  lost  to  our  means  of  detection. 

Can  we,  indeed,  suggest  a  proposition,  definitely  conceiv- 
able by  the  mind,  of  force  without  antecedent  force  ?  I  can- 
not, without  calling  for  the  interposition  of  created  power, 
any  more  than  I  can  conceive  the  sudden  appearance  of  a 


196  COEEELATION   OF   PHYSICAL   FORCES. 

mass  of  matter  come  from  nowhere,  and  formed  from  noth- 
ing. The  impossibility,  humanly  speaking,  of  creating  or 
annihilating  matter,  has  long  been  admitted,  though,  perhaps, 
its  distinct  reception  in  philosophy  may  be  set  down  to  the 
overthrow  of  the  doctrine  of  Phlogiston,  and  the  reformation 
of  chemistry  at  the  time  of  Lavoisier.  The  reasons  for  the 
admission  of  a  similar  doctrine  as  to  force  appear  to  be  equally 
strong.  With  regard  to  matter,  there  are  many  cases  in 
which  we  never  practically  prove  its  cessation  of  existence, 
yet  we  do  not  the  less  believe  in  it:  who,  for  instance,  can 
trace,  so  as  to  re-weigh,  the  •  particles  of  iron  worn  off 
the  tire  of  a  carriage  wheel?  who  can  re-combine  the  parti- 
cles of  wax  dissipated  and  chemically  changed  in  the  burning 
of  a  candle?  By  placing  matter  undergoing  physical  or 
chemical  changes  under  special  limiting  circumstances,  we 
may,  indeed,  acquire  evidence  of  its  continued  existence, 
weight  for  weight — and  so  we  may  in  some  instances  of  force, 
as  in  definite  electrolysis  :  indeed  the  evidence  we  acquire  of  the 
continued  existence  of  matter  is  by  the  continued  exertion  of 
the  force  it  exercises,  as,  when  we  weigh  it,  our  evidence  is 
the  force  of  attraction ;  so,  again,  our  evidence  of  force  is 
the  matter  it  acts  upon.  Thus,  matter  and  force  are  corre- 
lates, in  the  strictest  sense  of  the  word ;  the  conception  of 
the  existence  of  the  one  involves  the  conception  of  the  exis- 
tence of  the  other :  the  quantity  of  matter  again,  and  the  de- 
gree of  force,  involve  conceptions  of  space  and  time.  But 
to  follow  out  these  abstract  relations  would  lead  me  too  far 
into  the  alluring  paths  of  metaphysical  speculation. 

That  the  theoretical  portions  of  this  essay  are  open  to  ob- 
jection I  am  fully  conscious.  I  cannot,  however,  but  think 
that  the  fair  way  to  test  a  theory  is  to  compare  it  with  other 
theories,  and  to  see  whether  upon  the  whole  the  balance  of 
probability  is  in  its  favour.  Were  a  theory  open  to  no  ob- 
jection it  would  cease  to  be  a  theory,  and  become  a  law ; 
and  were  we  not  to  theorise,  or  to  take  generalised  views  of 


CONCLUDING   REMARKS.  197 

natural  phenomena  until  those  generalizations  were  sure  and 
unobjectionable — in  other  words,  were  laws — science  would 
be  lost  in  a  complex  mass  of  unconnected  observations, 
which  would  probably  never  disentangle  themselves.  Excess 
on  either  side  is  to  be  avoided ;  although  we  may  often  err  on 
the  side  of  hasty  generalisation,  we  may  equally  err  on  the 
side  of  mere  elaborate  collection  of  observations,  which, 
though  sometimes  leading  to  a  valuable  result,  yet,  when  cu- 
mulated without  a  connecting  link,  frequently  occasion  a  cost- 
ly waste  of  time,  and  leave  the  subject  to  which  they  refer  in 
greater  obscurity  than  that  in  which  it  was  involved  at  their 
commencement. 

Collections  of  facts  differ  in  importance,  as  do  theories : 
the  former,  in  many  instances,  derive  their  value  from  their 
capability  of  generalisation ;  while,  conversely,  theories  are 
valuable  as  methods  of  co-ordinating  given  series  of  facts, 
and  more  valuable  in  proportion  as  they  require  fewer  excep- 
tions and  fewer  postulates.  Facts  may  sometimes  be  as 
well  explained  by  one  view  as  by  another,  but  without  a 
theory  they  are  unintelligible  and  incommunicable.  Let  us 
use  our  utmost  effort  to  communicate  a  fact  without  using  the 
language  of  theory,  and  we  fail ;  theory  is  involved  in  all 
our  expressions  ;  the  knowledge  of  bygone  times  is  imported 
into  succeeding  times  by  terms  involving  theoretic  conceptions. 
As  the  knowledge  of  any  particular  science  developes  itself, 
our  views  of  it  become  more  simple  ;  hypotheses,  or  the  in- 
troduction of  supposititious  views,  are  more  and  more  dis- 
pensed with ;  words  become  applicable  more  directly  to  the 
phenomena,  and,  losing  the  hypothetic  meaning  which  they 
necessarily  possessed  at  their  reception,  acquire  a  secondary 
sense,  which  brings  more  immediately  to  our  minds  the  facts 
of  which  they  are  indices.  The  scaffolding  has  served  its 
purpose.  The  hypothesis  fades  away,  and  a  theory,  or  gen- 
eralised view  of  phenomena,  more  independent  of  supposition, 
but  still  full  of  gaps  and  difficulties,  takes  its  place.  This  in 


198  CORRELATION   OF   PHYSICAL   FORCES. 

its  turn,  should  the  science  continue  to  progress,  either  gives  place 
to  a  more  simple  and  wider  generalisation,  or  be  comes,  by  the  re- 
moval of  objections,  established  as  a  law.  Even  in  this  more 
advanced  stage,  words  importing  theory  must  be  used,  but 
phenomena  are  now  intelligible  and  connected,  though  express- 
ed by  varied  forms  of  speech. 

To  think  on  nature  is  to  theorise  ;  and  difficult  it  is  not 
to  be  led  on  by  the  continuities  of  natural  phenomena  to  the- 
ories which  appear  forced  and  unintelligible  to  those  who 
have  not  pursued  the  same  path  of  thought :  which,  more- 
over, if  allowed  to  gain  an  undue  influence,  seduce  us  from 
that  truth  which  is  the  sole  object  of  our  pursuit. 

Where  to  draw  the  line — where  to  say  thus  far  we  may 
go,  and  no  farther,  in  any  particular  class  of  analogies  or  re- 
lations which  Nature  presents  to  us  ;  how  far  to  follow  the 
progressive  indications  of  thought,  and  where  to  resist  its  al- 
lurements— is  a  question  of  degree  which  must  depend  upon 
the  judgment  of  each  individual  or  of  each  class  of  thinkers  ; 
yet  it  is  consolatory  that  thought  is  seldom  expended  in  vain. 

I  have  throughout  endeavoured  to  discard  the  hypotheses 
of  subtle  or  occult  entities  ;  if  in  this  endeavour  some  of  my 
views  have  been  adopted  upon  insufficient  data,  I  still  hope 
that  this  essay  will  not  prove  valueless. 

The  conviction  that  the  so-called  imponderables  are  modes 
of  motion,  will,  at  all  events,  lead  the  observer  of  natural 
phenomena  to  look  for  changes  in  these  affections,  wherever 
the  intimate  structure  of  matter  is  changed ;  and,  conversely, 
to  seek  for  changes  in  matter,  either  temporary  or  permanent, 
whenever  it  is  affected  by  these  forces.  I  believe  he  will 
seldom  do  this  in  vain.  It  was  not  until  I  had  long  reflected 
on  the  subject,  that  I  ventured  to  publish  my  views :  their 
publication  may  induce  others  to  think  on  their  subject-mat- 
ter. They  are  not  put  forward  with  the  same  objects,  nor  do 
they  aim  at  the  same  elaboration  of  detail,  as  memoirs  on 
newly-discovered  physical  facts :  they  purport  to  be  a  method 


CONCLUDING   KEMAKKS.  199 

of  mentally  regarding  known  facts,  some  few  of  which  I  have 
myself  made  known  on  other  occasions,  but  the  great  mass 
of  which  have  been  accumulated  by  the  labours  of  others, 
and  are  admitted  as  established  truths.  Every  one  has  a  right 
to  view  these  facts  through  any  medium  he  thinks  fit  to  em- 
ploy, but  some  theory  must  exist  in  the  minds  of  those  who 
reflect  upon  the  many  new  phenomena  which  have  recently, 
and  more  particularly  during  the  present  century,  been  dis- 
covered. It  is  by  a  generalised  or  connected  view  of  past 
acquisitions  in  natural  knowledge  that  deductions  can  best  be 
drawn  as  to  the  probable  character  of  the  results  to  be  antici- 
pated. It  is  a  great  assistance  in  such  investigations  to  be 
intimately  convinced  that  no  physical  phenomena  can  stand 
alone :  each  is  inevitably  connected  with  anterior  changes, 
and  as  inevitably  productive  of  consequential  changes,  each 
with  the  other,  and  all  with  time  and  space ;  and,  either  in 
tracing  back  these  antecedents  or  following  up  their  conse- 
quents, many  new  phenomena  will  be  discovered,  and 
many  existing  phenomena,  hitherto  believed  distinct,  will 
be  connected  and  explained :  explanation  is,  indeed,  only  re- 
lation to  something  more  familiar,  not  more  known — i.e. 
known  as  to  causative  or  creative  agencies.  In  all  pheno- 
mena the  more  closely  they  are  investigated  the  more  are  we 
convinced  that,  humanly  speaking,  neither  matter  nor  force 
can  be  created  or  annihilated,  and  that  an  essential  cause  is 
unattainable. — Causation  is  the  will,  Creation  the  act,  of 
God. 


NOTES   AND   REFERENCES. 


PAGE 

13.  THE  reader  who  is  curious  as  to  the  views  of  the  ancients,  regarding 

the  objects  of  science,  will  find  clues  to  them  in  the  second  book  of 
ARISTOTLE'S  Physics,  and  in  the  first  three  books  of  the  Metaphy- 
sics. See  also  the  Timaeus  of  PLATO,  and  BITTER'S  History  of 
Ancient  Philosophy,  where  a  sketch  of  the  Philosophy  of  LEUCIPPUS 
and  DEMOCRITUS  will  be  found. 

14.  BACON'S  Novum  Organum,  book  ii.  aph.  6  and  6. 

16  HUME'S  Enquiry  concerning  Human  Understanding,  S.  7,  London, 
1768. 

BROWN'S  Enquiry  into  the  Relations  of  Cause  and  Effect,  London, 
1835. 

The  illustration  I  have  used  of  floodgate  has  been  objected  to,  as 
being  one  to  which  the  term  cause  would  scarcely  be  applied,  but 
after  some  consideration  I  have  retained  it:  if  cause  be  viewed 
only  as  sequence,  it  must  be  limited  to  sequence  under  given  condi- 
tions or  circumstances,  and  here,  given  the  conditions,  the  sequence 
is  invariable.  I  see  no  difference  quoad  the  argument,  between 
this  illustration  and  that  of  BROWN  of  a  lighted  match  and  gun- 
powder (4th  edit.  p.  27),  to  which  my  reasoning  would  equally  well 
apply. 

HERSCHEL'S  Discourse  on  the  Study  of  Natural  Philosophy,  pp.  88 
and  149. 

17.  Quarterly  Review,  vol.  Ixviii.  p.  212. 

WHEWELL,  On  the  Question  '  Are  Cause  and  Effect  Successive  or 
Simultaneous?  (Cambridge  Philosophical  Transactions,  vol.  vii  p. 
319.) 

18.  HERSCHEL'S  Discourse,  p.  93. 

AMPERE,  Theorie  des  Phenomenes  Electro-dynamiques,  Memoirs  in 


NOTES   AND  REFERENCES.  201 


the  Ann,  de  Chimie  et  de  Physique,  and  works  from  1820  to  1826, 

Paris. 
23.  LAMAECK,  *  Sur  la  Matiere  du  Son '  (Journal  dc  Physique,  vol.  xlix. 

p.  397). 

25.  D'ALEMBERT,  Traite  de  Dynamique,  pp.  3  and  4,  Paris,  1796. 
28.  BABBAGE,  On  the  Permanent  Impression  of  our  Words  and  Actions  on 

the  Globe  we  inhabit,  9th  Bridgewater  Treatise,  ch.  ix. 
30.  MAYER,  Annalen  der  Pharmacie  Leibig  und  Wohler,  May  1852. 
33.  JOULE  on  the  Mechanical  Equivalent  of  Heat  (Phil.  Trans.  1850, 

p.  81.) 
83.  ERMAN,  Influence  of  Friction  upon  Thermo-electricity  (Reports  of  the 

British  Association,  1846.) 
36.  BECQUEREL,  Fegagement  de  1'Electricite  par  Frottement,  Traite  do 

1'Electricite",  torn.  ii.  p.  113  etseq. 

36.  SULLIVAN,  Currents  produced  by  the  vibration  of  metals  (Archiv.  de 

1'Electricite1,  t.  10,  p.  480).     LEROUX,  Vibrations  arrested  produce 
heat  (Cosmos,  March  30,  1860). 

37.  WHEATSTONE  on  the  Prismatic   Decomposition  of  Electrical  Light 

(Notices  of  Communications  to  the  British  Association,  p.  11, 

1835). 
39.  BACON,  De  Forma  Calidi,  Nov.  Org.  book  2,  aph.  20. 

RUMFORD,  An  Enquiry  concerning  the  Source  of  Heat  which  is  excited 

by  Friction  (Phil.  Trans,  p.  80,  1798). 
DAVY,  On  the  Conversion  of  Ice  into  Water  by  Friction  (West  of 

England  Contributions,  p.  16). 
Of  Heat  or  Calorific  Repulsion  (Elements  of  Chemical  Philosophy, 

p.  69). 

41.  BADEN  POWELL  on  the  Repulsive  Power  of  Heat  (Phil.  Trans.  1834, 

p.  486). 
FRESNEL,  Annales  de  Chimie,  torn*  xxix.  pp.  57  and  107. 

42.  MOSER  on  Invisible  Light  (Taylor's  Scientific  Memoirs,  vol.  iii.  pp. 

461  and  465). 

43.  BLACK  on  Latent  Heat  (Elements  of  Chemistry,  p.  144  et  passim, 

1803). 

45.  The  experiments  of  HENRY  and  DONNY  have  shown  that  the  cohesion 
of  liquids,  as  far  as  their  antagonism  to  rupture  goes,  is  much 
greater  than  has  been  generally  believed.  These  experiments, 
however,  make  no  difference  in  the  view  I  have  put  forth,  as,  what- 
ever be  the  character  of  the  attraction,  there  is  a  molecular  attrac- 
tion to  be  overcome  in  changing  bodies  from  the  solid  to  the  liquid 
state,  which  must  require  and  exhaust  force. 


202  NOTES  AND  REFERENCES. 

PAGE 

DONNY,  Sur  la  Cohesion  des  Liquides  (Memoires  de  1'Academie  Roy- 

ale  de  Bruxelles,  1843). 
HENRY,  Proceedings  of  the  American  Philosophical  Society,  April 

1844  (Silliman's  Journal,  vol.  xlviiii.  p.  215). 
48.  THILORIER,  Solidification  de  1'Acide  carbonique  (Ann.  de  Ch.  et  de 

Phys.  torn.  Ix.  p.  432). 

60.  I.  WEDGWOOD,  Thermometer  for  measuring  the  Higher  Degrees  of 

Heat  (Phil.  Trans.  1782,  p.  3Q5 ;  and  1785,  p.  390. 
TYNDALL,  on  the   physical   properties  of  Ice  (Phil.    Trans.  1858, 

p.  211). 
DESPRETZ,  Recherches  sur  le  Maximum  de  Densite  de  1'Eau  pure  et 

des  Dissolutions  aqueuses  (Ann.  de.  Ch.  et  de  Ph.  torn.  Ixx.  p.  45, 

and  torn.  Ixxiii.  p.  295). 

61.  BIOT  (Comptes  rendus  de  I'Academie  des  Sciences,  Paris  1850,   p. 

281).     The  experiments  on  circular  polarisation  by  water  were,  I 
believe,  by  Dr.  Leeson. 

62.  I.  THOMPSON,  Trans.  R.  S.  Edin.  vol.  xvi.  p.  575. 
W.  THOMPSON,  Phil.  Mag.  August  1850,  p.  123. 

BUNSEN,  Pogg.  Ann.  vol.  Ixxxi.  p.  562 ;  Ann.  de  Ch.  et  de  Phys.  vol. 
xxxv.  p.  383.  Effects  of  Pressure  on  the  Freezing  Point. 

63.  JOULE,  Phil.  Trans.  1852,  p.  99. 

Although,  taking  the  phenomena  as  they  are  known  to  exist,  the 
mechanical  laws  may  be  deduced,  yet  in  any  physical  conception 
of  the  nature  of  heat  the  expansion  by  cold  has  always  been  a 
great  stumbling-block  to  me,  and  I  believe  to  many  others. 

DULONG  and  PETIT,  and  REGNAULT.  See  their  Memoirs  abstracted 
and  referred  to  in  Gmelin's  Handbook  of  Chemistry,  translated  by 
Watts  for  the  Cavendish  Society,  vol.  i.  p.  242  et  seq. 

64.  WOOD,  Phil.  Mag.  1861,  1852. 

56.  SENARMONT,  Conduction  of  Heat  by  Crystals  (Gmelin's  Handbook  vol. 

i.  p.  222). 
66.  KNOBLAUCH,  Ann.  de  Ch.  et  de  Ph.  vol.  xxxvi.  p.  124. 

TYNDALL,  Transmission  of  Heat  through  Organic  Structures  (Phil. 

Trans,  vol.  cxliii.  p.  217). 

68.  GROVE,  Electricity  produced  by  approximating  Metals :  Report  of 
a  Lecture  at  the  London  Institution  (Literary  Gazette,  1843, 
p.  39). 

GASSIOT,  Phil.  Mag.  October  1844. 
ROGET,  On  the  Improbability  of  the  Contact  exciting  Force :  Treatise 

on  Galvanism  (Library  of  Useful  Knowledge,  S.  113). 
FARADAY,  Phil.  Trans.  1840,  p.  126. 


NOTES,  AND  REFERENCES.  203 

PAGE 

60.  MELLONI,  Sur  la  Polarisation  de  la  Chaleur :  Keeherches  sur  plusieurs 

Phenomenes  calorifiques  (Annales  de  Chimie  et  de  Ph.  torn.  xlv. 
pp.  5 — 68  ;  torn.  xli.  pp.  375-410  ;  torn,  xlviii.  pp.  198,  218). 
FORBES,  On  the  Refraction  and  Polarisation  of  Heat  (Transactions  of 
the  Royal  Society  of  Edinburgh,  vol.  xiii.  pp.  181,  168). 

61.  KIRCHOFF  Trans.  Belin  Acad.  1861. 

BALFOUR  STEWART  on  the  theory  of  Exchanges  (Report  British  Asso- 
ciation, 1861). 

63.  T.  WEDGWOOD,  On  the  Production  of  Light  and  Heat  by  different 
Bodies  (Phil.  Trans,  vol.  Ixxxii.  p.  272). 

65.  GROVE,  On  the  Decomposition  of  Water  into  its  Constituent  Gases  by 

Heat  (Phil.  Trans.  1847,  p.  1). 

ROBINSON,  On  the  Effect  of  Heat  hi  lessening  the  Affinities  of  the 
Elements  of  Water  (Transactions  of  the  Royal  Irish  Academy,  vol. 
xxi.  p.  2). 

67.  GROVE,  Water  decomposed  by  Chlorine  and  Heat  (Phil.  Trans.  1847, 
p.  20). 

70.  CARNOT,  Reflexions  sur  la  Puissance  motrice  du  Feu,  Paris,  1824. 

76.  SEGUIN,  Influence  des  Chemins  de  Fer,  p.  378  et  seq. 

77.  ROGERS,  Consumption  of  Coal  for  Man  power  (Cosmos,  vol.  ii.  p.  56). 

80.  Mr.  WATERSTON  has  suggested  that  solar  heat  may  arise  from  the 

mechanical  action  of  meteoric  stones  falling  into  the  sun,  and  Mr. 
THOMPSON  has  written  an  elaborate  paper  on  the  subject  (Trans. 
Brit.  Assoc.  1853).  If  a  number  of  gravitating  bodies  exist  hi  the 
neighbourhood  of  the  sun,  and  form,  as  is  conjectured,  the  zodia- 
cal light,  it  is  difficult  to  conceive  how  comets  as  they  approach 
this  region  steer  clear  of  such  bodies,  and  are  not  even  deflected 
from  their  orbits. 

For  Mr.  THOMPSON'S  various  and  valuable  papers,  see  Phil.  Mag.  1851 
to  1854  inclusive. 

81.  POISSON,  Comptes  rendus,  Paris,  January  30,  1837. 

83.  DUFAYE,  SYMMER,  WATSON,  and  FRANKLIN,  Theories  of  Electric  Fluid 

and  Electric  Fluids  (Priestley's  History  of  Electricity,  pp.  429 — 

441). 
83.  GROTTHUS,  Sur  la  Decomposition  de  1'Eau  et  des  Corps  qu'elle  tient 

en  dissolution  a  aide  de  l'Electricit6  galvanique  (Ann.  de  Chimie, 

torn.  Iviii.  p.  54). 
FARADAY,  On  the   Question  whether  Electrolytes  conduct  without 

Decomposition  (Proceedings  of  the  Weekly  Meetings  of  the  Royal 

Institution,  1866). 
GROVE  (Comptes  rendus,  Paris,  1839). 


204  NOTES  AND  KEFEKENCES. 

PAGE 

84.  FARADAY,  On  Induction  as  an  Action  of  contiguous  Particles  (Phil. 

Trans.  1838,  p.  30). 

86.  MATTEUCCI,  Plates  of  Mica  polarised  by  Electricity  (De  la  Hire's 

Electricity,  p.  140). 
GROVE,  Electrolysis  across  Glass  (Phil.  Mag.  Aug.  1860). 

85.  KARSTEN  on  Electrical  Figures  (Archiv.  de  1'Elec.  rols.  ii.  iii.  and  iv). 

87.  GROVE,  Etching  Electrical  Figures  and  transferring  them  to  Collo- 

dion (Phil.  Mag.  January  1857). 

88.  FUSINIERI,  Du  Transport  des  Matieres  ponderable  qui  s'opere  dans 

les  Decharges  electriques  (Archives  de  1'Electrieite  ;  Supplement  & 
la  Bibliotheque  universelle  de  Geneve,  torn.  iii.  p.  597). 

88.  GROVE,  On  the  Voltaic  Arc  (Report  of  Lecture  at  the  Royal  Insti- 
tution, Lit.  Gaz.  and  Athenaeum,  Feb.  7,  1845 ;  Phil.  Trans.  1847, 
p.  16). 

90  to  94.  GROVE,  On  the  Electro-chemical  Polarity  of  Gases  (Phil.  Trans. 
1852,  p.  87). 

94.  FREMY  and  E.  BECQUEREL,  Oxygen  changed  to  Ozone  by  the  Electric 
Spark  (Ann.  de  Oh.  et  de  Phys.  1852).  This  subject  and  the  na- 
ture of  Ozone  was  first  investigated  by  Dr.  Sehonbein.  See  also  a 
paper  by  Mr.  Brodie  On  the  Conditions  of  certain  Elements  at  the 
Moment  of  Chemical  Change  (Phil.  Trans.  1850). 

96,  96.  Molecular  Changes  in  Electrised  Metals  (NAIRNE,  Phil.  Trans. 
1780,  p.  334,  and  1793,  p.  223  ;  GROVE,  Electrical  Mag.  vol.  i.  p. 
120;  PELTIER,  Archives  de  1'Electricite,  vol.  v.  p.  182  ;  FUSINIERI, 
id.  p.  516). 

96.  WERTHEIM,  Change  in  Elasticity  of  Metals  by  Electrisation  (Ann.  de 

Ch.  et  de  Phys.  vol.  xii.  p.  623  ;  Arch.  Elec.  vol.  iv.  p.  490). 
DUFOUR,  Alteration  in  Tenacity  of  Metals  by  Electrisation  (Bibl.  univ. 
de  Geneve,  Fev.  1855,  p.  156). 

97.  MATTEUCCI,  Conduction  of  Electricity  by  Crystals  (Comptes  rendus  de 

1'Acad.,  Paris,  March,  5,  1855,  p.  541). 

98.  E.  BECQUEREL,  Transmission  of  Electricity  by  heated  Gases  (Ann,  de. 

Ch.  et  de  Phys.  vol.  xxxix.  p.  355). 
GROVE,  Proceedings  of  the  Royal  Inst.  (1854,  p.  361). 
BECQUEREL,  Divergence  of  Gold-Leaves  in  Vacuo  (Traite  d'Electricitd, 

vol.  v. ;  part  ii.  p.  53). 
NEWTON,  Thirty-first  Query  to  the  Optics. 

99.  GROVE,  Particles  of  Metals  and  Metallic  Oxids  detached  in  Liquids  by 

Electricity  (Elec.  Mag.  vol.  i.  p.  119). 
100.  MATTEUCCI,  Relations  of  Electricity  and  Nervous  Force  (Phil.  Trans. 


NOTES  AND  REFEEENCES.  205 


1845,  p.  -285,   1846,  p.  497;   Phenomenes  physiques  des  Corps 

vivants,  p.  305  ;  Lezioni  di  Fisica,  p.  360). 
GALVANI  YOLTA  MARIANINI  et  NOBILI  on  Physiological  Effects  of 

Electricity  (Ann.  de  Ch.  et  de  Phys.  vols.  23,  25,  29,  88,  40,  43, 

44,  56). 
102.  BECQUEREL,  Chemical  Changes  by  Friction  (Traite  de  1'Elec.  voL  v. 

part  1,  p.  16). 

106.  DE  LA  RIVE,  Heat  of  the  Voltaic  Pile  (Bibl.  univ.,  vol.  xiii.  p.  389). 
DAVY,  On  the  Properties  of  Electrified  Bodies  in  their  relations  to 

Conducting  Powers  and  Temperature  (Phil.  Trans.  1821,  p.  428). 

106.  GROVE,  On  the  Effects  of  surrounding  Media  on  Voltaic  Ignition  (Phil. 

Trans.  1849,  p.  49). 

107.  OERSTED,  Experience  sur  1'Effet  du  Conflict  e"lectrique  sur  1'Aiguille 

aimantee  (Ann.  de  Ch.  et  de  Phys.,  torn.  xiv.  p.  417). 

108.  COLERIDGE,  Table  Talk,  vol.  i.  p.  65. 

109.  LENZ  and  JACOBI,  Pogg.  Ann,  vol.  xvlii.  p.  403  ;  Bulletin  de  1'Acad. 

St.  Petersburg,  1839  ;  Harris,  Magnetism,  part  2,  p.  63. 

DAVY,  Decomposition  of  the  fixed  Alkalies  (Phil.  Trans.  1808,  p.  1). 

BECQUEREL  Des  Composes  e'lectrc-chimiques  (Trait6  de  1'Electricite, 
vol.  iii.  c.  13). 

CROSSE,  Transactions  of  the  British  Association,  vol.  v.  p.  47 ;  Pro- 
ceedings of  the  Electrical  Society,  p.  320. 

110.  MALUS,  Polarisation  of  Light  by  Reflection  (Me"moires  d'Arcueil,  torn. 

ii.  p.  143). 
ARAGO,  Circular  Polarisation  by  Solids  (Memoires  de  Tlnstitut,  1811). 

111.  BIOT,  Circular  Polarisation  by  Liquids  (Memoires  de  PInstitut,  1817). 
111.  NIEPCE  and  DAGUERRE,  Historique  et  Description  des  Precedes  du 

Daguerreotype,  Paris,  1839. 

TALBOT,  Photogenic  Drawing  and  Calotype  (Phil.  Mag.  March.  1839, 

and  August  1841). 

113.  HERSCHEL,  Chemical  Action  of  the  Solar  Spectrum  on  various  Sub- 
stances (Phil.  Trans.  1840,  p.  i.  and  1842,  p.  181). 

HUNT,  Researches  on  Light,  London,  1844. 

116.  GROVE,  Other  Forces  produced  by  Light  (Lit.  Gaz.  January  1844). 

117.  GROVE,  Influence  of  Light  on  the  Polarised  Electrode  (PhiL  Mag. 

December  1858). 

SOMERVILLE  (Mrs.),  On  the  Magnetising  Power  of  the  more  Refrangi- 
ble Solar  Rays  (Phil.  Trans.  1862,  p.  132). 

MORICHINI'S  experiments  are  given  in  Mrs.  Somerville's  paper. 

118.  HERSCHEL,  On  the  Absorption  of  Light  in  Coloured  Media  viewed 


206  NOTES   AND   REFERENCES. 


in  connection  with  the  Undulatory  Theory  (Phil.   Mag.  Decem- 
ber 1863). 
SEEBECK,  Heat  of  Coloured  Eays  (Brewster's  Optics,  p.  90). 

118.  KNOBLAUCH  (Ann.  de  Ch.  vol.  xxxvi.  p.  124,  and  Fogg.  Ann,  there 

referred  to). 

119.  HERSCHEL,  Epipolised  Light  (Phil.  Trans,  vol.  cxxxv.  pp.  143,  147). 
STOKES,  Change  in  Kefrangibility  of  Light  (Phil.  Trans,  vols.  cxlii. 

cxliii.) 

123.  For  the  first  enunciations  of  the  Corpuscular  and  Undulatory  Theories, 

see  NEWTON'S  Optics,  HOOKE'S  Micographia,  and  HUYGHENS'  Trac- 
tatus  de  Lumine.  See  also  BREWSTER'S  Optics,  p.  138. 

124.  YOUNG,  Lectures  edited  by  Kelland,  p.  358,  et  seq. ;  Phil.  Trans. 

1800,  p.  126;  HERSCHEL,  Encyc.  Metro,  art.  Light,  pp.  450  and 
738 ;  NEWTON'S  Optics,  p.  322 ;  WHEWELL'S  Hist.  Indue.  Sc.  vol. 
ii.  p.  449  ;  FOUCAULT,  Comptes  rendus,  Paris,  1850,  p.  65  ;  HARRI- 
SON, Phil.  Mag.  November  1856  ;  Camb.  Phil.  Trans. 

126.  SONDHAUSS,  Kefraction  of  Sound  (Ann.  de  Ch.  et  de  Phys.  vol. 
xxxv.  p.  605) ;  DOVE,  Polarisation  of  Sound  (Cosmos,  May  13, 
1859). 

182.  PASTEUR,  Rotation  of  Plane  of  Polarised  Light  by  Solutions  of 
Hemihedral  Crystals  (Ann.  de  Ch.  et  de  Phys.  vol.  xxiv.  p.  442). 

134  to  136.  WOLLASTON,  Phil.  Trans.  1822,  p.  89 ;  WHEWELL,  Phil,  of 
the  Induct.  Sc.  vol.  i.  p.  419  ;  WILSON,  Trans,  of  the  Roy.  Soc.  of 
Edin.  vol.  xvi.  p.  79 ;  Sir  W.  HERSCHEL,  Phil.  Trans.  1793,  p.  201, 
and  1801,  p.  300 ;  MORGAN,  Phil.  Trans,  vol.  Ixxv.  p.  272  ;  DAVY, 
Phil.  Trans.  1822,  p.  64 ;  Elements  of  Chemical  Philosophy,  p.  97 ; 
GASSIOT,  Phil.  Trans.  1859,  p.  157. 

187.  Diminishing  Periods  of  Comets  (Herschel's  Outlines  of  Astronomy, 
p.  357). 

140.  Since  writing  the  passage  in  the  text,  I  find  that  STRUVE  has  been 
led,  from  his  astronomical  researches,  to  the  conclusion  that  some 
light  is  lost  in  the  interplanetary  spaces.  He  gives  as  an  approxi- 
mation one  per  cent,  as  lost  by  the  passage  of  light  from  a  star  of 
the  first  magnitude,  assuming  a  mean  or  average  distance  (Etudes 
d' Astronomic  Stellaire,  1847). 
NEWTON,  Thirtieth  Query  to  the  Optics. 

142.  FARADAY,  Evolution  of  Electricity  from  Magnetism  (Phil.  Trans.* 
1882,  p.  126). 

144.  FARADAY,  Magnetic  Condition  of  all  Matter  (Phil.  Trans.  1846,  p.  21 ; 

Phil.  Mag.  1846,  p.  249). 

BECQUEREL,  Amy  de  Ch.  et  de  Ph.  torn,  xxxvi.  p.  337;  Comptes 
rendus,  Paris,  1846,  p.  147  ;  and  1850,  p.  201. 


NOTES   AND  KEFEKENCE8.  207 

PAGE 

145.  FARADAY,  On  the  Magnetism  of  Light  (Phil.  Trans.  1846,  p.  1). 

146.  WARTMANN,  Rotation  of  the  Plane  of  Polarisation  of  Heat  by  Magnet- 

ism (Journal  de  I'lnstitute,  No.  644). 
PROVOSTATE  and  DESSAINES,  Ann.  de  Ch.  et  de  Phys.  October  1849. 

146.  HUNT,  Influence  of  Magnetism  on  Molecular  Arrangement  (Phil.  Mag. 

1846,  vol.  xxviii.  p.  1 ;  Memoirs  of  the  Geological  Society,  vol.  i. 
p.  433). 
WARTMANN,  Phil.  Mag.  1847,  vol.  xxx.  p.  263. 

147.  GROVE,  Experiment  on  Molecular  Motion  of  a  Magnetic  Substance 

(Electrical  Mag.  1845,  vol.  i.  p.  601). 

147.  On  the  direct  Production  of  Heat  by  Magnetism  (Proceedings  of  the 

Royal  Society,  1849,  p.  826). 

After  this  paper  was  communicated  and  ordered  to  be  printed  in  the 
Philosophical  Transactions,  I  found  that  I  had  been  anticipated  by 
Mr.  VAN  BREDA,  who  communicated,  in  1845,  a  paper  to  the  Insti- 
tut  on  the  subject :  his  paper  appears  in  the  Comptes  rendus  under 
an  erroneous  title,  which  accounts  for  its  having  been  overlooked : 
he  does  not  give  thermometric  measures  of  the  heat  he  obtained, 
nor  did  he  produce  heating  effects  by  a  permanent  steel  magnet,  or 
with  other  metals  than  iron.  (Comptes  rendus,  October  27,  1845). 
See  also  an  earlier  experiment  by  Mr.  JOULE  (Phil.  Mag.  1843),  to 
which  he  called  my  attention  after  my  paper  was  read. 

148,  151.  The  Experiments  on  the  effects  of  Magnetism  on  the  Matter 

magnetised,  are  collected  by  Mr.  DE  LA  RIVE  hi  his  recently-pub- 
lished Treatise  on  Electricity,  vol.  i. 
163.  DAVY,  Electricity 'defined  as  Chemical  affinity  acting  on  Masses  (Phil. 

Trans.  1826,  p.  389). 

VOLTA,  Electricity  excited  by  the  mere  Contact  of  conducting  Sub- 
stances (Phil.  Trans.  1800,  p.  403). 

154.  GROVE,  Gold-Leaf  Experiment  (Comptes  rendus,  Paris,  1839,  p.  567). 
156.  GROVE,  Yoltaic  Action  of  Sulphur,  Phosphorus,  and  Hydrocarbons 

(Phil.  Trans.  1845,  p.  351). 

GROVE,  New  Yoltaic  Combination  (Phil.  Mag.  vol.  xiv.  p.  388 ;  vol. 
xv.  p.  287). 

155.  GROVE,   Electricity  of  Blowpipe  Flame  (Proceedings  of  the  Royal 

Institution,  February  1854),  Phil.  Mag. 

167.  DALTON,  New  System  of  Chemistry,  London,  1810. 

158.  I  have  here  and  elsewhere  used  whole  numbers,  as  sufficiently  approxi- 
mate for  the  argument,  but  without  intending  to  express  any  opin- 
ion as  to  the  law  of  PROUT. 

158.  FARADAY,  Definite  Electrolysis  (PhiL  Trans.  1834,  p.  77). 


208  NOTES  AND  REFERENCES. 

PA&E 

160.  WOOD,  Heat  disengaged  in  Chemical  Combinations  (Phil.  Mag.  1862). 

162.  ANDREWS,  Phil.  Trans.  1844,  p.  21. 

HESS,  PoggendoflPs  Annalen,  Bd.  lii.  p.  197. 

163.  FAVRE,  Ann,  de  Ch.  et  de  Phys.  vols.  39,  40  ;  Comptes  rendus,  Paris, 

v.ol.  45,  p.  56,  and  vol.  46,  p.  337. 

169.  CATALYSIS  by  Platinum  (DOBEREIMER,  Ann.  de  Ch.  et  de  Phys.  torn. 

xxiv.  p.  93  ;  DULONG  and  THENARD,  Ann,  de  Ch.  et  de  Phys.  torn, 
xxiii.  p.  440). 

170.  GROVE,  Gas  Voltaic  Battery  (Phil.  Mag.  February  1839,  and  Decem- 

ber 1842  ;  Phil.  Trans.  1843,  p.  91). 

171.  MOSOTTI,  Forces  which  regulate  the  Internal  Constitution  of  Bodies 

(Taylor's  Scientific  Memoirs,  vol.  i.  p.  448). 

172.  PLUCKER,  Repulsion  of  the  Optic  Axes  of  Crystals  by  the  Poles  of  a 

Magnet  (Taylor's  Scientific  Memoirs,  vol.  v.  p.  353). 
Magnetic  Action  of  Cyanite  (Lit.  Gaz.  1849,  p.  431). 

172.  MATTEUCCI,  Correlation  of  Electric  Current  and  Nervous  Force  (Phil. 

Trans.  1850,  p.  287). 

173.  CARPENTER,  On  the  Mutual  Relations  of  the  Vital  and  Physical  Forces 

(Phil.  Trans.  1850,  p.  751). 

174.  On  Effort.    See  BROWN,  Cause  and  Effect ;  HERSCHEL'S  Discourse  ; 

and  QUARTERLY  REVIEW,  June  1841. 

175.  HELMHOLTZ,  Muller's  Archives,  1845 ;  MATTEUCCI,  Comptes  rendus, 
Paris,  1856  ;  BECLARD,  Archives  de  Medicine,  1861. 

189.  DULONG  and  PETIT,  Relation  between  Specific  Heat  and  Chemical 

Equivalents  (Ann.  de  Ch.  et  de  Phys.  torn.  x.  p.  395). 
189.  NEUMANN,  PoggendorfTs  Annalen,  Bd.  xxiil.  p.  1. 
AVOGADRO,  Ann.  de  Ch.  et  de  Phys.  torn.  Iv.  p.  80. 


ON   THE 

ESTTEKACTION   OF  NATUKAL   FOKCES. 

BY  PEOF.  H.  L.  F.  HELMHOLTZ. 
TRANSLATED  BY   JOHN    TYNDALL,  F.R.S. 


HERMAN  LTJDWIG  FERDINAND  HELMHOLTZ  was  born  at  Pottsdam,  August 
31,  1821.  He  was  first  military  physician,  and  afterwards  assistant  of  the 
Astronomical  Museum  in  Berlin  (1848),  and  subsequently  Professor  Extra- 
ordinary of  Physiology  at  the  University  of  Konigsberg  (1849  to  1852).  He 
became  Professor  of  Physiology  at  the  University  of  Bonn  in  1855,  and  in 
1858  accepted  the  physiological  chair  in  the  University  of  Heidelberg.  The 
lecture  which  follows  was  delivered  at  Konigsberg  hi  1854.  He  is  an  emi- 
nent investigator,  and  an  able  promoter  of  the  recent  philosophy  of  forces  ; 
but  of  his  life  we  have  fewer  particulars  than  of  his  accomplished  translator. 

The  ancestors  of  JOHN  TYNDALL  emigrated  from  England  to  the  eastern 
or  Saxon  border  of  Ireland  about  the  middle  of  the  last  century.  He  was 
born  at  the  village  of  Leighlin  Bridge  in  1820,  where  he  received  his  early 
education  and  acquired  a  taste  for  mathematics.  In  1839  he  left  school 
and  joined  the  Ordnance  Survey  as  a  civil  assistant,  where  he  became  in 
turn  draughtsman,  computer,  surveyor,  and  trigometrical  observer.  He  was 
five  years  connected  with  the  survey,  and  for  three  years  occupied  as  rail- 
road engineer.  In  1847  he  became  teacher  in  Queenswood  College  in  Hamp- 
shire, a  school  for  agriculturists  and  engineers,  where  he  was  distinguished 
for  his  mild  but  efficient  discipline.  Professor  Frankland,  the  chemist,  was 
here  joined  with  him  hi  the  work  of  instruction,  and  in  1848  the  two  friends 
left  the  institution  and  went  to  the  University  of  Marburg  in  Hesse  Cassel, 
to  study  with  the  eminent  chemist,  Bunsen.  In  1851  Professor  Tyndall 
went  to  Berlin  and  worked  at  the  subject  of  diamagnetism  in  the  laboratory 
of  Professor  Magnus.  He  returned  to  London  the  same  year,  and  was 
elected  Fellow  of  the  Eoyal  Society  in  1852. 

Through  the  influence  of  Dr.  Bence  Jones,  General  Sabine,  and  Professor 
Faraday,  he  was  appointed  Professor  of  Natural  Philosophy  in  the  Royal 
Institution  in  1853,  an  appointment  which  he  now  holds.  In  company  with 
his  friend,  Professor  Huxley,  he  visited  the  Alps  hi  1856 ;  and  returning  each 
succeeding  year,  he  accumulated  the  observations  and  adventures  which  are 
so  graphically  described  in  his  "Glaciers  of  the  Alps,"  published  in  1860. 
Professor  Tyndall  has  worked  with  eminent  success  at  various  scientific 
questions,  but  he  is  chiefly  distinguished  for  his  original  and  elaborate  re- 
searches on  the  relations  of  radiant  heat  to  gaseous  and  vaporous  matter. 
These  researches  are  given  hi  his  able  work  on  "  Heat  as  a  mode  of  Motion," 
issued  hi  1863.  As  an  experimenter,  Professor  Tyndall  is  marked  for  his 
caution,  accuracy,  and  tireless  perseverance  under  difficulties ;  as  a  writer, 
for  his  clear,  vivid,  and  vigorous  style. 


INTERACTION  OF  NATURAL  FORCES. 


ANEW  conquest  of  very  general  interest  has  been  recently 
made  by  natural  philosophy.  In  the  following  pages,  I 
will  endeavour  to  give  a  notion  of  the  nature  of  this  conquest. 
It  has  reference  to  a  new  and  universal  natural  law,  which 
rules  the  action  of  natural  forces  in  their  mutual  relations 
towards  each  other,  and  is  as  influential  on  our  theoretic 
views  of  natural  processes  as  it  is  important  in  their  technical 
applications. 

Among  the  practical  arts  which  owe  their  progress  to  the 
development  of  the  natural  sciences,  from  the  conclusion  of 
the  middle  ages  downwards,  practical  mechanics,  aided  by 
the  mathematical  science  which  bears  the  same  name,  was 
one  of  the  most  prominent.  The  character  of  the  art  was,  at 
the  time  referred  to,  naturally  very  different  from  its  present 
one.  Surprised  and  stimulated  by  its  own  success,  it  thought 
no  problem  beyond  its  power,  and  immediately  attacked  some 
of  the  most  difficult  and  complicated.  Thus  it  was  attempted 
to  build  automaton  figures  which  should  perform  the  functions 
of  men  and  animals.  The  wonder  of  the  last  century  was 
Vaucanson's  duck,  which  fed  and  digested  its  food ;  the  flute- 
player  of  the  same  artist,  which  moved  all  its  fingers  cor- 


212  INTERACTION  OF  NATURAL  FORCES. 

rectly  ;  the  writing  boy  of  the  older,  and  the  piano-forte  play- 
er of  the  younger  Droz  :  which  latter,  when  performing,  fol- 
lowed its  hands  with  his  eyes,  and  at  the  conclusion  of  the 
piece  bowed  courteously  to  the  audience.  That  men  like 
those  mentioned,  whose  talent  might  bear  comparison  with 
the  most  inventive  heads  of  the  present  age,  should  spend  so 
much  time  in  the  construction  of  these  figures,  which  we  at 
present  regard  as  the  merest  trifles,  would  be  incomprehensi- 
ble, if  they  had  not  hoped  in  solemn  earnest  to  solve  a  great 
problem.  The  writing  boy  of  the  elder  Droz  was  publicly 
exhibited  in  Germany  some  years  ago.  Its  wheel-work  is  so 
complicated,  that  no  ordinary  head  would  be  sufficient  to 
decipher  its  manner  of  action.  When,  however,  we  are  in- 
formed that  this  boy  and  its  constructor,  being  suspected  of  the 
black  art,  lay  for  a  time  in  the  Spanish  Inquisition,  and  with 
difficulty  obtained  their  freedom,  we  may  infer  that  in  those 
days  even  such  a  toy  appeared  great  enough  to  excite  doubts 
as  to  its  natural  origin.  And  though  these  artists  may  not 
have  hoped  to  breathe  into  the  creature  of  their  ingenuity  a 
soul  gifted  with  moral  completeness,  still  there  were  many 
who  would  be  willing  to  dispense  with  the  moral  qualities  of 
their  servants,  if,  at  the  same  time,  their  immoral  qualities 
could  also  be  got  rid  of ;  and  accept,  instead  of  the  mutability 
of  flesh  and  bones,  services  which  should  combine  the  regu- 
larity of  a  machine  with  the  durability  of  brass  and  steel. 
The  object,  therefore,  which  the  inventive  genius  of  the  past 
century  placed  before  it  with  the  fullest  earnestness,  and  not 
as  a  piece  of  amusement  merely,  was  boldly  chosen,  and  was 
followed  up  with  an  expenditure  of  sagacity  which  has  contri- 
buted not  a  little  to  enrich  the  mechanical  experience  which  a 
later  time  knew  how  to  take  advantage  of.  We  no  longer 
seek  to  build  machines  which  shall  fulfil  the  thousand  services 
required  of  one  man,  but  desire,  on  the  contrary,  that  a  ma- 
chine shall  perform  one  service,  but  shall  occupy  in  doing  it 
the  place  of  a  thousand  men. 


THE  OLD  MECHANICAL  PROBLEM.         213 

From  these  efforts  to  imitate  living  creatures,  another  idea, 
also  by  a  misunderstanding,  seems  to  have  developed  itself, 
which,  as  it  were,  formed  the  new  philosopher's  stone  of  the 
seventeenth  and  eighteenth  centuries.  It  was  now  the  endeav- 
our to  construct  a  perpetual  motion.  Under  this  term  was  un- 
derstood a  machine,  which,  without  being  wound  up,  without 
consuming  in  the  working  of  it,  falling  water,  wind,  or  any 
other  natural  force,  should  still  continue  in  motion,  the  motive 
power  being  perpetually  supplied  by  the  machine  itself.  Beasts 
and  human  beings  seemed  to  correspond  to  the  idea  of  such  an 
apparatus,  for  they  moved  themselves  energetically  and  inces- 
santly as  long  as  they  lived,  were  never  wound  up,  and  nobody 
set  them  in  motion.  A  connection  between  the  taking-in  of 
nourishment  and  the  development  of  force  did  not  make  itself 
apparent.  The  nourishment  seemed  only  necessary  to  grease, 
as  it  were,  the  wheel  work  of  the  animal  machine,  to  replace 
what  was  used  up,  and  to  renew  the  old.  The  development 
of  force  out  of  itself  seemed  to  be  the  essential  peculiarity,  the 
real  quintessence  of  organic  life.  If,  therefore,  men  were  to 
be  constructed,  a  perpetual  motion  must  first  be  found. 

Another  hope  also  seemed  to  take  up  incidentally  the  sec- 
ond place,  which,  in  our  wiser  age,  would  certainly  have 
claimed  the  first  rank  in  the  thoughts  of  men.  The  perpetual 
motion  was  to  produce  work  inexhaustibly  without  corre- 
sponding consumption,  that  is  to  say,  out  of  nothing.  "Work, 
however,  is  money.  Here,  therefore,  the  practical  problem 
which  the  cunning  heads  of  all  centuries  have  followed  in  the 
most  diverse  ways,  namely,  to  fabricate  money  out  of  nothing, 
invited  solution.  The  similarity  with  the  philosopher's  stone 
sought  by  the  ancient  chemists  was  complete.  That  also 
was  thought  to  contain  the  quintessence  of  organic  life,  and  to 
be  capable  of  producing  gold. 

The  spur  which  drove  men  to  inquiry  was  sharp,  and  the 
talent  of  some  of  the  seekers  must  not  be  estimated  as  small. 
The  nature  of  the  problem  was  quite  calculated  to  entice  por- 


214  INTERACTION  OF  NATURAL  FOECES. 

ing  brains,  to  lead  them  round  a  circle  for  years,  deceiving 
ever  with  new  expectations,  which  vanished  upon  nearer  ap- 
proach, and  finally  reducing  these  dupes  of  hope  to  open  in- 
sanity. The  phantom  could  not  be  grasped.  It  would  be 
impossible  to  give  a  history  of  these  efforts,  as  the  clearer 
heads,  among  whom  the  elder  Droz  must  be  ranked,  convinced 
themselves  of  the  futility  of  their  experiments,  and  were 
naturally  not  inclined  to  speak  much  about  them.  Bewildered 
intellects,  however,  proclaimed  often  enough  that  they  had 
discovered  the  grand  secret ;  and  as  the  incorrectness  of  their 
proceedings  was  always  speedily  manifest,  the  matter  fell  into 
bad  repute,  and  the  opinion  strengthened  itself  more  and  more 
that  the  problem  was  not  capable  of  solution ;  one  difficulty 
after  another  was  brought  under  the  dominion  of  mathemati- 
cal mechanics,  and  finally  a  point  was  reached  where  it  could 
be  proved,  that,  at  least  by  the  use  of  pure  mechanical  forces, 
no  perpetual  motion  could  be  generated. 

We  have  here  arrived  at  the  idea  of  the  driving  force  or 
power  of  a  machine,  and  shall  have  much  to  do  with  it  in 
future.  I  must,  therefore,  give  an  explanation  of  it.  The 
idea  of  work  is  evidently  transferred  to  machines  by  compar- 
ing their  arrangements  with  those  of  men  and  animals  to 
replace  which  they  were  applied.  "We  still  reckon  the  work 
of  steam  engines  according  to  horse-power.  The  value  of 
manual  labor  is  determined  partly  by  the  force  which  is  ex- 
pended in  it  (a  strong  laborer  is  valued  more  highly  than  a 
weak  one) ,  partly  however,  by  the  skill  which  is  brought  into 
action.  A  machine,  on  the  contrary,  which  executes  work 
skilfully,  can  always  be  multiplied  to  any  extent ;  hence  its 
skill  has  not  the  high  value  of  human  skill  in  domains  where 
the  latter  cannot  be  supplied  by  machines.  Thus  the  idea  of 
the  quantity  of  work  in  the  case  of  machines  has  been  limited 
to  the  consideration  of  the  expenditure  of  force  ;  this  was  the 
more  important,  as  indeed  most  machines  are  constructed  for 
the  express  purpose  of  exceeding,  by  the  magnitude  of  their 


_^b  CLEMENT   OF  MECHANICAL   POWER.  215 

effects,  the  powers  of  men  and  animals.  Hence,  in  a  mechani- 
cal sense,  the  idea  of  work  is  become  identical  with  that  of 
the  expenditure  of  force,  and  in  this  way  I  will  apply  it. 

How,  then,  can  we  measure  this  expenditure,  and  compare 
it  in  the  case  of  different  machines  ? 

I  must  here  conduct  you  a  portion  of  the  way — as  short  a 
portion  as  possible — over  the  uninviting  field  of  mathematico- 
mechanical  ideas,  in  order  to  bring  you  to  a  point  of  view  from 
which  a  more  rewarding  prospect  will  open.  And  though  the 
example  which  I  shall  here  choose,  namely,  that  of  a  water- 
mill  with  iron  hammer,  appears  to  be  tolerably  romantic,  still, 
alas,  I  must  leave  the  dark  forest  valley,  the  spark-emitting 
anvil,  and  the  black  Cyclops  wholly  out  of  sight,  and  beg  a 
moment's  attention  to  the  less  poetic  side  of  the  question, 
namely,  the  machinery.  This  is  driven  by  a  water-wheel  which 
in  its  turn  is  set  in  motion  by  the  falling  water.  The  axle  of  the 
water-wheel  has  at  certain  places  small  projections,  thumbs, 
which,  during  the  rotation,  lift  the  heavy  hammer  and  permit 
it  to  fall  again.  The  falling  hammer  belabors  the  mass  of 
metal,  which  is  introduced  beneath  it.  The  work  therefore 
done  by  the  machine  consists,  in  this  case,  in  the  lifting  of  the 
hammer,  to  do  which  the  gravity  of  the  latter  must  be  over- 
come. The  expenditure  of  force  will,  in  the  first  place,  other 
circumstances  being  equal,  be  proportioned  to  the  weight  of 
the  hammer  ;  it  will,  for  example,  be  double  when  the  weight 
of  the  hammer  is  doubled.  But  the  action  of  the  hammer 
depends  not  upon  its  weight  alone,  but  also  upon  the  height 
from  which  it  falls.  If  it  falls  through  two  feet,  it  will  pro- 
duce a  greater  effect  than  if  it  falls  through  only  one  foot.  It 
is,  however,  clear  that  if  the  machine,  with  a  certain  expendi- 
ture of  force,  lifts  the  hammer  a  foot  in  height,  the  same 
amount  of  force  must  be  expended  to  raise  it  a  second  foot  in 
height.  The  work  is  therefore  not  only  doubled  when  the 
weight  of  the  hammer  is  increased  twofold,  but  also  when  the 
space  through  which  it  falls  is  doubled.  From  this  it  is  easy 


216         INTERACTION  OFNATURAL  FORCES. 

to  see  that  the  work  must  be  measured  by  the  product  of  the 
weight  into  the  space  through  which  it  ascends.  And  in  this 
way,  indeed,  do  we  measure  in  mechanics. 

The  unit  of  work  is  a  foot-pound,  that  is,  a  pound  weight 
raised  to  the  height  of  one  foot. 

While  the  work  in  this  case  consists  in  the  raising  of  the 
heavy  hammer-head,  the  driving  force  which  sets  the  latter  in 
motion,  is  generated  by  falling  water.  It  is  not  necessary 
that  the  water  should  fall  vertically,  it  can  also  flow  in  a 
moderately  inclined  bed ;  but  it  must  always,  where  it  has 
water-mills  to  set  in  motion,  move  from  a  higher  to  a  lower 
position.  Experiment  and  theory  coincide  in  teaching,  that 
when  a  hammer  of  a  hundred  weight  is  to  be  raised  one  foot, 
to  accomplish  this  at  least  a  hundred  weight  of  water  must 
fall  through  the  space  of  one  foot ;  or  what  is  equivalent  to 
this,  two  hundred  weight  must  fall  full  half  a  foot,  or  four  hun- 
dred weight  a  quarter  of  a  foot,  etc.  In  short,  if  we  multiply 
the  weight  of  the  falling  water  by  the  height  through  which  it 
falls,  and  regard,  as  before,  the  product  as  the  measure  of  the 
work,  then  the  work  performed  by  the  machine  in  raising  the 
hammer,  can,  in  the  most  favourable  case,  be  only  equal  to  the 
number  of  foot-pounds  of  water  which  have  fallen  in  the  same 
time.  In  practice,  indeed,  this  ratio  is  by  no  means  attained  ; 
a  great  portion  of  the  work  of  the  falling  water  escapes  unused, 
inasmuch  as  part  of  the  force  is  willingly  sacrificed  for  the 
sake  of  obtaining  greater  speed. 

I  will  further  remark,  that  this  relation  remains  unchanged 
whether  the  hammer  is  driven  immediately  by  the  axle  of  the 
wheel,  or  whether — by  the  intervention  of  wheel-work,  end- 
less screws,  pulleys,  ropes — the  motion  is  transferred  to  the 
hammer.  We  may,  indeed,  by  such  arrangements,  succeed 
in  raising  a  hammer  of  ten  hundred  weight,  when  by  the  first 
simple  arrangement,  the  elevation  of  a  hammer  of  one  hundred 
weight  might  alone  be  possible  ;  but  either  this  heavier  ham- 
mer is  raised  to  only  one  tenth  of  the  height,  or  tenfold  tho 


TETTE  FUNCTION   OF  MACHINES.  217 

time  is  required  to  raise  it  to  the  same  height ;  so  that,  how- 
ever we  may  alter,  by  the  interposition  of  machinery,  the  in- 
tensity of  the  acting  force,  still  in  a  certain  time,  during  which 
the  mill-stream  furnishes  us  with  a  definite  quantity  of  water, 
a  certain  definite  quantity  of  work,  and  no  more,  can  be  per- 
formed. 

Our  machinery,  therefore,  has,  in  the  first  place,  done 
nothing  more  than  make  use  of  the  gravity  of  the  falling  wa- 
ter in  order  to  overpower  the  gravity  of  the  hammer,  and  to 
raise  the  latter.  When  it  has  lifted  the  hammer  to  the  neces- 
sary height,  it  again  liberates  it,  and  the  hammer  falls  upon 
the  metal  mass  which  is  pushed  beneath  it.  But  why  does 
the  falling  hammer  here  exercise  a  greater  force  than  when  it 
is  permitted  simply  to  press  with  its  own  weight  on  the  mass 
of  metal  ?  Why  is  its  power  greater  as  the  height  from  which 
it  falls  is  increased  ?  We  find,  in  fact,  that  the  work  per- 
formed by  the  hammer  is  determined  by  its  velocity.  In 
other  cases,  also,  the  velocity  of  moving  masses  is  a  means  of 
producing  great  eifects.  I  only  remind  you  of  the  destructive 
effects  of  musket-bullets,  which,  in  a  state  of  rest,  are  the  most 
harmless  things  in  the  world.  I  remind  you  of  the  wind-mill, 
which  derives  its  force  from  the  moving  air.  It  may  appear 
surprising  that  motion,  which  we  are  accustomed  to  regard  as 
a  non-essential  and  transitory  endowment  of  bodies,  can  pro- 
duce such  great  effects.  But  the  fact  is,  that  motion  appears 
to  us,  under  ordinary  circumstances,  transitory,  because  the 
movement  of  all  terrestrial  bodies  is  resisted  perpetually  by 
other  forces,  friction,  resistance  of  the  air,  etc.,  so  that  motion 
is  incessantly  weakened  and  finally  neutralized.  A  body, 
however,  which  is  opposed  by  no  resisting  force,  when  once 
set  in  motion,  moves  onward  eternally  with  undiminished 
velocity.  Thus  we  know  that  the  planetary  bodies  have 
moved  without  change,  through  space,  for  thousands  of  years. 
Only  by  resisting  forces  can  motion  be  diminished  or  destroyed. 
A  moving  body,  such  as  the  hammer  or  the  musket-ball,  when 
10 


218  INTERACTION  OF  NATURAL  FORCES. 

it  strikes  against  another,  presses  the  latter  together,  or  pene- 
trates it,  until  the  sum  of  the  resisting  forces  which  the  body 
struck  presents  to  its  pressure,  or  to  the  separation  of  its  par- 
ticles, is  sufficiently  great  to  destroy  the  motion  of  the  ham- 
mer or  of  the  bullet.  The  motion  of  a  mass  regarded  as 
taking  the  place  of  working  force  is  called  the  living  force  (vis 
viva)  of  the  mass.  The  word  "  living  "  has  of  course  here 
no  reference  whatever  to  living  beings,  but  is  intended  to  rep- 
resent solely  the  force  of  the  motion  as  distinguished  from  the 
state  of  unchanged  rest — from  the  gravity  of  a  motionless 
body,  for  example,  which  produces  an  incessant  pressure 
against  the  surface  which  supports  it,  but  does  not  produce 
any  motion. 

In  the  case  before  us,  therefore,  we  had  first  power  in  the 
form  of  a  falling  mass  of  water,  then  in  the  form  of  a  lifted 
hammer,  and,  thirdly,  in  the  form  of  the  living  force  of  the 
fallen  hammer.  We  should  transform  the  third  form  into  the 
second,  if  we,  for  example,  permitted  the  hammer  to  fall  upon 
a  highly  elastic  steel  beam  strong  enough  to  resist  the  shock. 
The  hammer  would  rebound,  and  in  the  most  favourable  case 
would  reach  a  height  equal  to  that  from  which  it  fell,  but 
would  never  rise  higher.  In  this  way  its  mass  would  ascend : 
and  at  the  moment  when  its  highest  point  has  been  attained, 
it  would  represent  the  same  number  of  raised  foot-pounds  as 
before  it  fell,  never  a  greater  number  ;  that  is  to  say,  living 
force  can  generate  the  same  amount  of  work  as  that  ex- 
pended in  its  production.  It  is  therefore  equivalent  to  this 
quantity  of  work. 

Our  clocks  are  driven  by  means  of  sinking  weights,  and 
our  watches  by  means  of  the  tension  of  springs.  A  weight 
which  lies  on  the  ground,  an  elastic  spring  which  is  without 
tension,  can  produce  no  effects  ;  to  obtain  such  we  must  first 
raise  the  weight  or  impart  tension  to  the  spring,  which  is 
accomplished  when  we  wind  up  our  clocks  and  watches. 
The  man  who  winds  the  clock  or  watch  communicates  to  the 


RESERVOIR  OF  ACCUMULATED   POWER.  219 

weight  or  to  the  spring  a  certain  amount  of  power,  and  ex- 
actly so  much  as  is  thus  communicated  is  gradually  given  out 
again  during  the  following  twenty-four  hours,  the  original 
force  being  thus  slowly  consumed  to  overcome  the  friction  of 
the  wheels  and  the  resistance  which  the  pendulum  encounters 
from  the  air.  The  wheel-work  of  the  clock  therefore  exhibits 
no  working  force  which  was  not  previously  communicated  to 
it,  but  simply  distributes  the  force  given  to  it  uniformly  over 
a  longer  time. 

Into  the  chamber  of  an  air-gun  we  squeeze,  by  means  of 
a  condensing  air-pump,  a  great  quantity  of  air.  When  we 
afterwards  open  the  cock  of  a  gun  and  admit  the  compressed 
air  into  the  barrel,  the  ball  is  driven  out  of  the  latter  with  a 
force  similar  to  that  exerted  by  ignited  powder.  Now  we 
may  determine  the  work  consumed  in  the  pumping-in  of  the 
air,  and  the  living  force  which,  upon  firing,  is  communicated 
to  the  ball,  but  we  shall  never  find  the  latter  greater  than  the 
former.  The  compressed  air  has  generated  no  working  force, 
but  simply  gives  to  the  bullet  that  which  has  been  previously 
communicated  to  it.  And  while  we  have  pumped  for  perhaps 
a  quarter  of  an  hour  to  charge  the  gun,  the  force  is  expended 
in  a  few  seconds  when  the  bullet  is  discharged ;  but  because 
the  action  is  compressed  into  so  short  a  time,  a  much  greater 
velocity  is  imparted  to  the  ball  than  would  be  possible  to  com- 
municate to  it  by  the  unaided  effort  of  the  arm  in  throw- 
ing it. 

•From  these  examples  you  observe,  and  the  mathematical 
theory  has  corroborated  this  for  all  purely  mechanical,  that  is 
to  say,  for  moving  forces,  that  all  our  machinery  and  appara- 
tus generate  no  force,  but  simply  yield  up  the  power  com- 
municated to  them  by  natural  forces, — falling  water,  moving 
wind,  or  by  the  muscles  of  men  and  animals.  After  this  law 
had  been  established  by  the  great  mathematicians  of  the  last 
century,  a  perpetual  motion,  which  should  make  only  use  of 
pure  mechanical  forces,  such  as  gravity,  elasticity,  pressure  of 


220  INTERACTION   OF  NATURAL   FORCES. 

liquids  and  gases,  could  only  be  sought  after  by  bewildered 
and  ill-instructed  people.  But  there  are  still  other  natural 
forces  which  are  not  reckoned  among  the  purely  moving 
forces, — heat,  electricity,  magnetism,  light,  chemical  forces, 
all  of  which  nevertheless  stand  in  manifold  relation  to  me- 
chanical processes.  There  is  hardly  a  natural  process  to  be 
found  which  is  not  accompanied  by  mechanical  actions,  or 
from  which  mechanical  work  may  not  be  derived.  Here  the 
question  of  a  perpetual  motion  remained  open  ;  the  decision 
of  this  question  marks  the  progress  of  modern  physics. 

In  the  case  of  the  air-gun,  the  work  to  be  accomplished  in 
the  propulsion  of  the  ball  was  given  by  the  arm  of  the  man 
who  pumped  in  the  air.  In  ordinary  firearms,  the  condensed 
mass  of  air  which  propels  the  bullet  is  obtained  in  a  totally 
different  manner,  namely,  by  the  combustion  of  the  powder. 
Gunpowder  is  transformed  by  combustion  for  the  most  part 
into  gaseous  products,  which  endeavor  to  occupy  a  much 
larger  space  than  that  previously  taken  up  by  the  volume  of 
the  powder.  Thus,  you  see,  that,  by  the  use  of  gunpowder, 
the  work  which  the  human  arm  must  accomplish  in  the  case 
of  the  air-gun  is  spared. 

In  the  mightiest  of  our  machines,  the  steam  eagine,  it  is  a 
strongly  compressed  aeriform  body,  water  vapour,  which,  by 
its  effort  to  expand,  sets  the  machine  in  motion.  Here  also, 
we  do  not  condense  the  steam  by  means  of  an  external 
mechanical  force,  but  by  communicating  heat  to  .a  mass  of 
water  in  a  closed  boiler,  we  change  this  water  into  steam, 
which,  in  consequence  of  the  limits  of  the  space,  is  developed 
under  strong  pressure.  In  this  case,  therefore,  it  is  the  heat 
communicated  which  generates  the  mechanical  force.  The 
heat  thus  necessary  for  the  machine  we  might  obtain  in  many 
ways  ;  the  ordinary  method  is  to  procure  it  from  the  combus- 
tion of  coal. 

Combustion  is  a  chemical  process.  A  particular  constitu- 
ent of  our  atmosphere,  oxygen,  possesses  a  strong  force  of 


PRODUCTION   OF   FOKCE   BY   COMBUSTION.  221 

attraction,  or,  as  it  is  named  in  chemistry,  a  strong  affinity 
for  the  constituents  of  the  combustible  body,  which  affinity, 
however,  in  most  cases,  can  only  exert  itself  at  high  tempera- 
tures. As  soon  as  a  portion  of  the  combustible  body,  for  ex- 
ample the  coal,  is  sufficiently  heated,  the  carbon  unites  itself 
with  great  violence  to  the  oxygen  of  the  atmosphere  and  forms 
a  peculiar  gas,  carbonic  acid,  the  same  which  we  see  foaming 
from  beer  and  champagne.  By  this  combination,  light  and  heat 
are  generated ;  heat  is  generally  developed  by  any  combination 
of  two  bodies  of  strong  affinity  for  each  other ;  and  when  the 
heat  is  intense  enough,  light  appears.  Hence,  in  the  steam 
engine,  it  is  chemical  processes  and  chemical  forces  which  pro- 
duce the  astonishing  work  of  these  machines.  In  like  manner 
the  combustion  of  gunpowder  is  a  chemical  process,  which, 
in  the  barrel  of  the  gun,  communicates  living  force  to  the 
bullet. 

While  now  the  steam  engine  develops  for  us  mechanical 
work  out  of  heat,  we  can  conversely  generate  heat  by  mechani- 
cal forces.  A  skilful  blacksmith  can  render  an  iron  wedge  red 
hot  by  hammering.  The -axles  of  our  carriages  must  be  pro- 
tected "by  careful  greasing,  from  ignition  through  friction. 
Even  lately  this  property  has  been  applied  on  a  large  scale.  In 
some  factories,  where  a  surplus  of  water  power  is  at  hand,  this 
surplus  is  applied  to  cause  a  strong  iron  plate  to  rotate  swiftly 
upon  another,  so  that  they  become  strongly  heated  by  the  fric- 
tion. The  heat  so  obtained  warms  the  room,  and  thus  a  stove 
without  fuel  is  provided.  Now,  could  not  the  heat  generated 
by  the  plates  be  applied  to  a  small  steam  engine,  which,  in  its 
turn,  should  be  able  to  keep  the  rubbing  plates  in  motion  ? 
The  perpetual  motion  would  thus  be  at  length  found.  This 
question  might  be  asked,  and  could  not  be  decided  by  the 
older  mathematico-mechanical  investigations.  I  will  remark, 
beforehand,  that  the  general  law  which  I  will  lay  before  you 
answers  the  question  in  the  negative. 

By  a  similar  plan,  however,  a  speculative  American  set 


222  INTERACTION   OF  NATURAL   FORCES. 

some  time  ago  the  industrial  world  of  Europe  in  excitement. 
The  magneto- electric  machines  often  made  use  of  in  the  case 
of  rheumatic  disorders  are  well  known  to  the  public.  By 
imparting  a  swift  rotation  to  the  magnet  of  such  a  machine, 
we  obtain  powerful  currents  of  electricity.  If  those  be  con- 
ducted through  water,  the  latter  will  be  reduced  into  its  two 
components,  oxygen  and  hydrogen.  By  the  combustion  of 
hydrogen,  water  is  again  generated.  If  this  combustion  takes 
place,  not  in  atmospheric  air,  of  which  oxygen  only  consti- 
tutes a  fifth  part,  but  in  pure  oxygen,  and  if  a  bit  of  chalk  be 
placed  in  the  flame,  the  chalk  will  be  raised  to  a  white  heat, 
and  give  us  the  sun-like  Drummond's  light.  At  the  same 
time,  the  flame  develops  a  considerable  quantity  of  heat. 
Our  American  proposed  to  utilize  in  this  way  the  gases 
obtained  from  electrolytic  decomposition,  and  asserted  that  by 
the  combustion  a  sufficient  amount  of  heat  was  generated  to 
keep  a  small  steam  engine  in  action,  which  again  drove  his 
magneto-electric  machine,  decomposed  the  water,  and  thus 
continually  prepared  its  own  fuel.  This  would  certainly  have 
been  the  most  splendid  of  all  discoveries  ;  a  perpetual  motion 
which,  besides  the  force  which  kept  it  going,  generated  light 
like  the  sun,  and  warmed  all  around  it.  The  matter  was  by 
no  means  badly  cogitated.  Each  practical  step  in  the  affair 
was  known  to  be  possible  ;  but  those  which  at  that  time  were 
acquainted  with  the  physical  investigations  which  bear  upon 
this  subject  could  have  affirmed,  on  the  first  hearing  the 
report,  that  the  .matter  was  to  be  numbered  among  the  numer- 
ous stories  of  the  fable-rich  America ;  and  indeed,  a  fable  it 
remained. 

It  is  not  necessary  to  multiply  examples  further.  You 
will  infer  from  those  given,  in  what  immediate  connection 
heat,  electricity,  magnetism,  light,  and  chemical  affinity,  stand 
with  mechanical  forces. 

Starting  from  each  of  these  different  manifestations  of 
natural  forces,  we  can  set  every  other  in  motion,  for  the  most 


STATEMENT   OF  DYNAMIC   PROBLEM.  223 

part  not  in  one  way  merely,  but  in  many  ways.     It  is  here  as 
with  the  weaver's  web, — 

Where  a  step  stirs  a  thousand  threads, 

The  shuttles  shoot  from  side  to  side, 

The  fibres  flow  unseen, 

And  one  shock  strikes  a  thousand  combinations. 

Now  it  is  clear  that  if  by  any  means  we  could  succeed,  as 
the  above  American  professed  to  have  done,  by  mechanical 
forces,  to  excite  chemical,  electrical,  or  other  natural  pro- 
cesses, which,  by  any  circuit  whatever,  and  without  altering 
permanently  the  active  masses  in  the  machine,  could  produce 
mechanical  force  in  greater  quantity  than  that  at  first  applied, 
a  portion  of  the  work  thus  gained  might  be  made  use  of  to 
keep  the  machine  in  motion,  while  the  rest  of  the  work  might 
be  applied  to  any  other  purpose  whatever.  The  problem 
was,  to  find  in  the  complicated  net  of  reciprocal  actions,  a 
track  through  chemical,  electrical,  magnetical,  and  thermic 
processes,  back  to  mechanical  actions,  which  might  be  followed 
with  a  final  gain  of  mechanical  work ;  thus  would  the  perpet- 
ual motion  be  found. 

But,  warned  by  the  futility  of  former  experiments,  the 
public  had  become  wiser.  On  the  whole,  people  did  not  seek 
much  after  combinations  which  promised  to  furnish  a  perpetual 
motion,  but  the  question  was  inverted.  It  was  no  more 
asked,  How  can  I  make  use  of  the  known  and  unknown  rela- 
tions of  natural  forces  so  as  to  construct  a  perpetual  motion  ? 
but  it  was  asked,  If  a  perpetual  motion  be  impossible,  what 
are  the  relations  which  must  subsist  between  natural  forces  ? 
Everything  was  gained  by  this  inversion  of  the  question. 
The  relations  of  natural  forces  rendered  necessary  by  the 
above  assumption,  might  be  easily  and  completely  stated.  It 
was  found  that  all  known  relations  of  force  harmonize  with 
the  consequences  of  that  assumption,  and  a  series  of  unknown 
relations  were  discovered  at  the  same  time,  the  correctness  of 


224:  INTERACTION   OF  NATURAL   FORCES. 

which  remained  to  be  proved.  If  a  single  one  of  them  could 
be  proved  false,  then  a  perpetual  motion  would  be  possible. 

The  first  who  endeavoured  to  travel  this  way  was  a  French- 
man, named  Carnot,  in  the  year  1824.  In  spite  of  a  too 
limited  conception  of  his  subject,  and  an  incorrect  view  as  to 
the  nature  of  heat,  which  led  him  to  some  erroneous  conclu- 
sions, his  experiment  was  not  quite  unsuccessful.  He  dis- 
covered a  law  which  now  bears  his  name,  and  to  which  I  will 
return  further  on. 

His  labors  remained  for  a  long  time  without  notice,  and  it 
was  not  till  eighteen  years  afterwards,  that  is,  in  1842,  that 
different  investigators  in  different  countries,  and  independent 
of  Carnot,  laid  hold  of  the  same  thought. 

The  first  who  saw  truly  the  general  law  here  referred  to, 
and  expressed  it  correctly,  was  a  German  physician,  J.  R. 
Mayer,  of  Heilbronn,  in  the  year  1842.  A  little  later,  in 
1843,  a  Dane,  named  Colding,  presented  a  memoir  to  the 
Academy  of  Copenhagen,  in  which  the  same  law  found  utter- 
ance, and  some  experiments  were  described  for  its  further 
corroboration.  In  England,  Joule  began  about  the  same  time 
to  make  experiments  having  reference  to  the  same  subject. 
We  often  find,  in  the  case  of  questions  to  the  solution  of 
which  the  development  of  science  points,  that  several  heads, 
quite  independent  of  each  other,  generate  exactly  the  same 
series  of  reflections. 

I  myself,  without  being  acquainted  with  either  Mayer  or 
Colding,  and  having  first  made  the  acquaintance  of  Joule's 
experiments  at  the  end  of  my  investigation,  followed  the  same 
path.  I  endeavoured  to  ascertain  all  the  relations  between  the 
different  natural  processes,  which  followed  from  our  regarding 
them  from  the  above  point  of  view.  My  inquiry  was  made 
public  in  1847,  in  a  small  pamphlet  bearing  the  title,  u  On 
the  Conservation  of  Force." 

Since  that  time  the  interest  of  the  scientific  public  for  this 
subject  has  gradually  augmented.  A  great  number  of  the 


PKOGKESS   OF  THE   INVESTIGATION.  225 

essential  consequences  of  the  above  manner  of  viewing  the 
subject,  the  proof  of  which  was  wanting  when  the  first 
theoretic  notions  were  published,  have  since  been  confirmed 
by  experiment,  particularly  by  those  of  Joule  ;  and  during 
the  last  year  the  most  eminent  physicist  of  France,  Regnault, 
has  adopted  the  new  mode  regarding  the  question,  and  by 
fresh  investigations  on  the  specific  heat  of  gases  has  contri- 
buted much  to  its  support.  For  some  important  consequences 
the  experimental  proof  is  still  wanting,  but  the  number  of 
confirmations  is  so  predominant,  that  I  have  not  deemed  it 
too  early  to  bring  the  subject  before  even  a  non-scientific 
audience. 

How  the  question  has  been  decided  you  may  already  infer 
from  what  has  been  stated.  In  the  series  of  natural  processes 
there  is  no  circuit  to  be  found,  by  which  mechanical  force  can 
be  gained  without  a  corresponding  consumption.  The  per- 
petual motion  remains  impossible.  Our  reflections,  however, 
gain  thereby  a  higher  interest. 

We  have  thus  far  regarded  the  development  of  force  by 
natural  processes,  only  in  its  relation  to  its  usefulness  to  man, 
as  mechanical  force.  You  now  see  that  we  have  arrived  at  a 
general  law,  which  holds  good  wholly  independent  of  the 
application  which  man  makes  of  natural  forces ;  we  must 
therefore  make  the  expression  of  our  new  law  correspond  to 
this  more  general  significance.  It  is  in  the  first  place  clear, 
that  the  work  which,  by  any  natural  process  whatever,  is  per- 
formed under  favourable  conditions  by  a  machine,  and  which 
may  be  measured  in*  the  way  already  indicated,  may  be  used 
as  a  measure  of  force  common  to  all.  Further,  the  impor- 
tant question  arises,  "  If  the  quantity  of  force  cannot  be  aug- 
mented except  by  corresponding  consumption,  can  it  be 
diminished  or  lost?  For  the  purpose  of  our  machines  it  cer- 
tainly can,  if  we  neglect  the  opportunity  to  convert  natural 
processes  to  use,  but  as  investigation  has  proved,  not  for  a, 
nature  as  a  whole." 
10* 


226  INTERACTION   OF  NATURAL   FOKCES. 

In  the  collision  and  friction  of  bodies  against  each  other, 
the  mechanics  of  former  years  assumed  simply  that  living 
force  was  lost.  But  I  have  already  stated  that  each  collision 
and  each  act  of  friction  generates  heat ;  and,  moreover,  Joule 
has  established  by  experiment  the  important  law,  that  for 
every  foot-pound  of  force  which  is  lost,  a  definite  quantity  of 
heat  is  always  generated,  and  that  when  work  is  performed 
by  the  consumption  of  heat,  for  each  foot-pound  thus  gained  a 
definite  quantity  of  heat  disappears.  The  quantity  of  heat 
necessary  to  raise  the  temperature  of  a  pound  of  water  a  de- 
gree of  the  centigrade  thermometer,  corresponds  to  a  mechani- 
cal force  by  which  a  pound  weight  would  be  raised  to  the 
height  of  1350  feet ;  we  name  this  quantity  the  mechanical 
equivalent  of  heat.  I  may  mention  here  that  these  facts  con- 
duct of  necessity  to  the  conclusion,  that  the  heat  is  not,  as 
was  formerly  imagined,  a  fine  imponderable  substance,  but 
that,  like  light,  it  is  a  peculiar  shivering  motion  of  the  ulti- 
mate particles  of  bodies.  In  collision  and  friction,  according 
to  this  manner  of  viewing  the  subject,  the  motion  of  the  mass 
of  a  body  which  is  apparently  lost  is  converted  into  a  motion 
of  the  ultimate  particles  of  the  body ;  and  conversely,  when 
mechanical  force  is  generated  by  heat,  the  motion  of  the  ulti- 
mate particles  is  converted  into  a  motion  of  the  mass. 

Chemical  combinations  generate  heat,  and  the  quantity  of 
this  heat  is  totally  independent  of  the  time  and  steps  through 
which  the  combination  has  been  effected,  provided  that  other 
actions  are  not  at  the  same  time  brought  into  play.  If,  however, 
mechanical  work  is  at  the  same  time  accomplished,  as  in  the 
case  of  the  steam  engine,  we  obtain  as  much  less  heat  as  is 
equivalent  to  this  work.  The  quantity  of  work  produced  by 
chemical  force  is  in  general  very  great.  A  pound  of  the 
purest  coal  gives,  when  burnt,  sufficient  heat  to  raise  the  tem- 
perature of  8086  pounds  of  water  one  degree  of  the  centi- 
grade thermometer  ;  from  this  we  can  calculate  that  the  mag- 
nitude of  the  chemical  force  of  attraction  between  the  parti- 


AMOUNT  OF  FORCE  IN  THE  UNIVERSE  UNALTERABLE.    227 

cles  of  a  pound  of  coal  and  the  quantity  of  oxygen  that  corre- 
sponds to  it,  is  capable  of  lifting  a  weight  of  one  hundred 
pounds  to  a  height  of  twenty  miles.  Unfortunately,  in  our 
steam  engines,  we  have  hitherto  been  able  to  gain  only  the 
smallest  portion  of  this  work ;  the  greater  part  is  lost  in  the 
shape  of  heat.  The  best  expansive  engines  give  back  as 
mechanical  work  only  eighteen  per  cent,  of  the  heat  generated 
by  the  fuel. 

From  a  similar  investigation  of  all  the  other  known  physi- 
cal and  chemical  processes,  we  arrive  at  the  conclusion  that 
Nature  as  a  whole  possesses  a  store  of  force  which  cannot  in 
any  way  be  either  increased  or  diminished.  And  that,  there- 
fore, the  quantity  of  force  in  nature  is  just  as  eternal  and 
unalterable  as  the  quantity  of  matter.  Expressed  in  this  form, 
I  have  named  the  general  law  "  The  Principle  of  the  Conser- 
vation of  Force." 

We  cannot  create  mechanical  force,  but  we  may  help  our- 
selves from  the  general  store-house  of  Nature.  The  brook 
and  the  wind,  which  drive  our  mills,  the  forest  and  the  coal- 
bed,  which  supply  our  steam  engines  and  warm  our  rooms, 
are  to  us  the  bearers  of  a  small  portion  of  the  great  natural 
supply  which  we  draw  upon  for  our  purposes,  and  the  actions 
of  which  we  can  apply  as  we  think  fit.  The  possessor  of  a 
mill  claims  the  gravity  of  the  descending  rivulet,  or  the  living 
force  of  the  moving  wind,  as  his  possession.  These  por- 
tions of  the  store  of  Nature  are  what  give  his  property  its 
chief  value. 

Further,  from  the  fact  that  no  portion  of  force  can  be 
absolutely  lost,  it  does  not  follow  that  a  portion  may  not  be 
inapplicable  to  human  purposes.  In  this  respect  the  infer- 
ences drawn  by  William  Thomson  from  the  law  of  Carnot 
are  of  importance.  This  law,  which  was  discovered  by  Car- 
not during  his  endeavours  to  ascertain  the  relations  between 
heat  and  mechanical  force,  which,  however,  by  no  means 
belongs  to  the  necessary  consequences  of  the  conservation  of 


228  INTERACTION  OF  NATURAL  FORCES. 

force,  and  which  Clausius  was  the  first  to  modify  in  such  a 
manner  that  it  no  longer  contradicted  the  above  general  law, 
expresses  a  certain  relation  between  the  compressibility,  the 
capacity  for  heat,  and  the  expansion  by  heat  of  all  bodies.  It 
is  not  yet  considered  as  actually  proved,  buf  some  remarkable 
deductions  having  been  drawn  from  it,  and  afterwards  proved 
to  be  facts  by  experiment,  it?  has  attained  thereby  a  great 
degree  of  probability.  Besides  the  mathematical  form  in 
which  the  law  was  first  expressed  by  Carnot,  we  can  give  it 
the  following  more  general  expression : — "  Only  when  heat 
passes  frqm  a  warmer  to  a  colder  body,  and  even  then  only 
partially,  can  it  be  converted  into  mechanical  work." 

The  heat  of  a  body  which  we  cannot  cool  further,  cannot 
be  changed  into  another  form  of  force ;  into  the  electric  or 
chemical  force,  for  example.  Thus,  in  our  steam  engines, 
we  convert  a  portion  of  the  heat  of  the  glowing  coal  into 
work,  by  permitting  it  to  pass  to  the  less  warm  water  of  the 
boiler.  If,  however,  all  the  bodies  in  nature  had  the  same 
temperature,  it  would  be  impossible  to  convert  any  portion  of 
their  heat  into  mechanical  work.  According  to  this,  we  can 
divide  the  total  force  store  of  the  universe  into  two  parts,  one 
of  which  is  heat,  and  must  continue  to  be  such ;  the  other,  to 
which  a  portion  of  the  heat  of  the  warmer  bodies,  and  the 
total  supply  of  chemical,  mechanical,  electrical,  and  magneti- 
cal  forces  belong,  is  capable  of  the  most  varied  changes  of 
form,  and  constitutes  the  whole  wealth  of  change  which  takes 
place  in  nature. 

But  the  heat  of  the  warmer  bodies  strives  perpetually  to 
pass  to  bodies  less  warm  by  radition  and  conduction,  and  thus 
to  establish  an  equilibrium  of  temperature.  At  each  motion 
of  a  terrestrial  body,  a  portion  of  mechanical  force  passes  by 
friction  or  collision  into  heat,  of  which  only  a  part  can  be 
converted  back  again  into  mechanical  force.  This  is  also 
generally  the  case  in  every  electrical  and  chemical  process. 
From  this,  it  follows  that  the  first  portion  of  the  store  of  force, 


THE  FORCES  OF  NATUKE  DISSIPATED  IN  HEAT.         229 

the  unchangeable  heat,  is  augmented  by  every  natural  pro- 
cess, while  the  second  portion,  mechanical,  electrical,  and 
chemical  force,  must  be  diminished ;  so  that  if  the  universe 
be  delivered  over  to  the  undisturbed  action  of  its  physical  pro- 
cesses, all  force  will  finally  pass  into  the  form  of  heat,  and  all 
heat  come  into  a  state  of  equilibrium.  Then  all  possibility  of 
a  further  change  would  be  at  an  end,  and  the  complete  cessa- 
tion of  all  natural  processes  must  set  in.  The  life  of  men, 
animals,  and  plants,  could  not  of  course  continue  if  the  sun 
had  lost  its  high  temperature,  and  with  it  his  light, — if  all  the 
components  of  the  earth's  surface  had  closed  thos.e  combina- 
tions which  their  affinities  demand.  In  short,  the  universe 
from  that  time  forward  would  be  condemned  to  a  state  of 
eternal  rest. 

These  consequences  of  the  law  of  Carnot  are,  of  course, 
only  valid,  provided  that  the  law,  when  sufficiently  tested, 
proves  to  be  universally  correct.  In  the  mean  time  there  is 
little  prospect  of  the  law  being  proved  incorrect.  At  all 
events  we  must  admire  the  sagacity  of  Thomson,  who,  in  the 
letters  of  a  long  known  little  mathematical  formula,  which 
only  speaks  of  the  heat,  volume,  and  pressure  of  bodies, 
was  able  to  discern  consequences  which  threatened  the  uni- 
verse, though  certainly  after  an  infinite  period  of  time,  with 
eternal  death. 

I  have  already  given  you  notice  that  our  path  lay  through 
a  thorny  and  unrefreshing  field  of  mathematico-mechanical 
developments.  We  have  now  left  this  portion  of  our  road 
behind  us.  The  general  principle  which  I  have  sought  to  lay 
before  you  has  conducted  us  to  a  point  from  which  our  view 
is  a  wide  one,  and,  aided  by  this  principle,  we  can  now  at 
pleasure  regard  this  or  the  other  side  of  the  surrounding 
world,  according  as  our  interest  in  the  matter  leads  us.  A 
glance  into  the  narrow  laboratory  of  the  physicist,  with  its 
small  appliances  and  complicated  abstractions,  will  not  be  so 
attractive  as  a  glance  at  the  wide  heaven  above  us,  the  clouds, 


230  INTERACTION   OF   NATURAL   FORCES. 

the  rivers,  the  woods,  and  the  living  beings  around  us.  While 
regarding  the  laws  which  have  been  deduced  from  the  physi- 
cal processes  of  terrestrial  bodies,  as  applicable  also  to  the 
heavenly  bodies,  let  me  remind  you  that  the  same  force  which, 
acting  at  the  earth's  surface,  we  call  gravity  (Schwere),  acts 
as  gravitation  in  the  celestial  spaces,  and  also  manifests  its 
power  in  the  motion  of  the  immeasurably  distant  double  stars 
which  are  governed  by  exactly  the  same  laws  as  those  sub- 
sisting between  the  earth  and  moon ;  that,  therefore,  the 
light  and  heat  of  terrestrial  bodies  do  not  in  any  way  differ 
essentially  from  those  of  the  sun,  or  of  the  most  distant  fixed 
star  ;  that  the  meteoric  stones  which  sometimes  fall  from  ex- 
ternal space  upon  the  earth  are  composed  of  exactly  the  same 
simple  chemical  substances  as  those  with  which  we  are 
acquainted.  We  need,  therefore,  feel  no  scruple  in  granting 
that  general  laws  to  which  all  terrestrial  natural  processes 
are  subject,  are  also  valid  for  other  bodies  than  the  earth. 
We  will,  therefore,  make  use  of  our  law  to  glance  over  the 
household  of  the  universe  with  respect  to  the  store  of  force, 
capable  of  action,  which  it  possesses. 

A  number  of  singular  peculiarities  in  the  structure  of  our 
planetary  system  indicate  that  it  was  once  a  connected  mass 
with  a  uniform  motion  of  rotation.  Without  such  an  assump- 
tion, it  is  impossible  to  explain  why  all  the  planets  move  in  the 
same  direction  round  the  sun,  why  they  all  rotate  in  the  same 
direction  round  their  axes,  why  the  planes  of  their  orbits,  and 
those  of  their  satellites  and  rings  all  nearly  coincide,  why  all 
their  orbits  differ  but  little  from  circles,  and  much  besides. 
From  these  remaining  indications  of  a  former  state,  astrono- 
mers have  shaped  an  hypothesis  regarding  the  formation  of 
our  planetary  system,  which,  although  from  the  nature  of  the 
case  it  must  ever  remain  an  hypothesis,  still  in  its  special 
traits  is  so  well  supported  by  analogy,  that  it  certainly  de- 
serves our  attention.  It  was  Kant,  who,  feeling  great  inter- 
est in  the  physical  description  of  the  earth  and  the  planetary 


ORIGIN   OF  THE  NEBULAR   HYPOTHESIS.  231 

system,  undertook  the  labour  of  studying  the  works  of  New- 
ton, and  as  an  evidence  of  the  depth  to  which  he  had  pene- 
trated into  the  fundamental  ideas  of  Newton,  seized  the  notion 
that  the  same  attractive  force  of  all  ponderable  matter  which 
now  supports  the  motion  of  the  planets,  must  also  aforetime 
have  been  able  to  form  from  matter  loosely  scattered  in  space 
the  planetary  system.  Afterwards,  and  independent  of  Kant, 
Laplace,  the  great  author  of  the  Mecanique  Celeste,  laid  hold 
of  the  same  thought,  and  introduced  it  among  astronomers. 

The  commencement  of  our  planetary  system,  including 
the  sun,  must,  according  to  this,  be  regarded  as  an  immense 
nebulous  mass  which  filled  the  portion  of  space  which  is  now 
occupied  by  our  system,  far  beyond  the  limits  of  Neptune, 
our  most  distant  planet.  Even  now  we  perhaps  see  similar 
masses  in  the  distant  regions  of  the  firmament,  as  patches  of 
nebulas,  and  nebulous  stars  ;  within  our  system  also,  comets, 
the  zodiacal  light,  the  corona  of  the  sun  during  a  total  eclipse, 
exhibit  remnants  of  a  nebulous  substance,  which  is  so  thin 
that  the  light  of  the  stars  passes  through  it  unenfeebled  and 
unrefracted.  If  we  calculate  the  density  of  the  mass  of  our 
planetary  system,  according  to  the  above  assumption,  for  the 
time  when  it  was  a  nebulous  sphere,  which  reached  to  the 
path  of  the  outmost  planet,  we  should  find  that  it  would 
require  several  cubic  miles  of  such  matter  to  weigh  a  single 
grain. 

The  general  attractive  force  of  all  matter  must,  however, 
impel  these  masses  to  approach  each  other,  and  to  condense, 
so  that  the  nebulous  sphere  became  incessantly  smaller,  by 
which,  according  to  mechanical  laws,  a  motion  of  rotation 
originally  slow,  and  the  existence  of  which  must  be  assumed, 
would  gradually  become  quicker  and  quicker.  By  the  cen- 
trifugal force  which  must  act  most  energetically  in  the  neigh- 
bourhood of  the  equator  of  the  nebulous  sphere,  masses 
could  from  time  to  time  be  torn  away,  which  afterwards  would 
continue  their  courses  separate  from  the  main  mass,  forming 


232  INTERACTION  OF  NATURAL  FORCES. 

themselves  into  single  planets,  or,  similar  to  the  great  origi- 
nal sphere,  into  planets  with  satellites  and  rings,  until  finally 
the  principal  mass  condensed  itself  into  the  sun.  With 
regard  to  the  origin  of  heat  and  light,  this  view  gives  us  no 
information. 

When  the  nebulous  chaos  first  separated  itself  from  other 
fixed  star  masses,  it  must  not  only  have  contained  all  kinds 
of  matter  which  was  to  constitute  the  future  planetary  sys- 
tem, but  also,  in  accordance  with  our  new  law,  the  whole 
store  of  force  whicji  at  one  time  must  unfold  therein  its  wealth 
of  actions.  Indeed  in  this  respect  an  immense  dower  was 
bestowed  in  the  shape  of  the  general  attraction  of  all  the  par- 
ticles for  each  other.  This  force,  which  on  the  earth  exerts 
itself  as  gravity,  acts  in  the  heavenly  spaces  as  gravitation. 
As  terrestrial  gravity  when  it  draws  a  weight  downwards 
performs  work  and  generates  vis  viva,  so  also  the  heavenly 
bodies  do  the  same  when  they  draw  two  portions  of  matter 
from  distant  regions  of  space  towards  each  other. 

The  chemical  forces  must  have  been  also  present,  ready 
to  act ;  but  as  these  forces  can  only  come  into  operation 
by  the  most  intimate  contact  of  the  different  masses,  con- 
densation must  have  taken  place  before  the  play  of  chemical 
forces  began. 

Whether  a  still  further  supply  of  force  in  the  shape  of 
heat  was  present  at  the  commencement  we  do  not  know.  At 
all  events,  by  aid  of  the  law  of  the  equivalence  of  heat  and 
work,  we  find  in  the  mechanical  forces,  existing  at  the  time 
to  which  we  refer,  such  a  rich  source  of  heat  and  light,  that 
there  is  no  necessity  whatever  to  take  refuge  in  the  idea  of  a, 
store  of  these  forces  originally  existing.  When  through  con- 
densation of  the  masses  their  particles  came  into  collision, 
and  clung  to  each  other,  the  vis  viva  of  their  motion  would  be 
thereby  annihilated,  and  must  reappear  as  heat.  Already  in 
old  theories,  it  has  been  calculated,  that  cosmical  masses  must 
generate  heat  by  their  collision,  but  it  was  far  from  any  body's 


HEAT   DEVELOPED   IN   THE   SOLAR   SYSTEM.  233 

thought,  to  make  even  a  guess  at  the  amount  of  heat  to  be 
generated  in  this  way.  At  present  we  can  give  definite 
numerical  values  with  certainty. 

Let  us  make  this  addition  to  our  assumption  ;  that,  at  the 
commencement,  the  density  of  the  nebulous  matter  was  a  van- 
ishing quantity,  as  compared  with  the  present  density  of  the 
sun  and  planets  ;  we  can  then  calculate  how  much  work  has 
been  performed  by  the  condensation ;  we  can  further  calcu- 
late how  much  of  this  work  still  exists  in  the  form  of  mechani- 
cal force,  as  attraction  of  the  planets  towards  the  sun,  and  as 
vis  viva  of  their  motion,  and  find,  by  this,  how  much  of  the 
force  has  been  converted  into  heat. 

The  result  of  this  calculation  is,  that  only  about  the  454th 
part  of  the  original  mechanical  force  remains  as  such,  and 
that  the  remainder,  converted  into  heat,  would  be  sufficient  to 
raise  a  mass  of  water  equal  to  the  sun  and  planets  taken  to- 
gether, not  less  than  twenty-eight  millions  of  degrees  of  the 
centigrade  scale.  For  the  sake  of  comparison,  I  will  mention 
that  the  highest  temperature  which  we  can  produce  by  the 
oxyhydrogen  blowpipe,  which  is  sufficient  to  fuse  and  vapor- 
ize even  platina,  and  which  but  few  bodies  can  endure,  is 
estimated  at  about  two  thousand  centigrade  degrees.  Of  the 
action  of  a  temperature  of  twenty-eight  millions  of  such  de- 
grees we  can  form  no  notion.  If  the  mass  of  our  entire  sys- 
tem were  pure  coal,  by  the  combustion  of  the  whole  of  it  only 
the  3500th  part  of  the  above  quantity  would  be  generated. 
This  is  also  clear,  that  such  a  development  of  heat  must  have 
presented  the  greatest  obstacle  to  the  speedy  union  of  the 
masses,  that  the  larger  part  of  the  heat  must  have  been 
diffused  by  radiation  into  space,  before  the  masses  could  form 
bodies  possessing  the  present  density  of  the  sun  and  planets, 
and  that  these  bodies  must  once  have  been  in  a  state  of  fiery 
fluidity.  This  notion  is  corroborated  by  the  geological  phe- 
nomena of  our  planet ;  and  with  regard  to  the  other  planetary 
bodies,  the  flattened  form  of  the  sphere,  which  is  the  form  of 


234:  INTERACTION   OF   NATURAL   FORCES. 

equilibrium  of  a  fluid  mass,  is  indicative  of  a  former  state  of 
fluidity.  If  I  thus  permit  an  immense  quantity  of  heat 
to  disappear  without  compensation  from  our  system,  the 
principle  of  the  conservation  of  force  is  not  thereby  invaded. 
Certainly  for  our  planet  it  is  lost,  but  not  for  the  universe. 
It  has  proceeded  outwards,  and  daily  proceeds  outwards  into 
infinite  space  ;  and  we  know  not  whether  the  medium  which 
transmits  the  undulations  of  light  and  heat  possesses  an  end 
where  the  rays  must  return,  or  whether  they  eternally  pursue 
their  way  through  infinitude. 

The  store  of  force  at  present  possessed  by  our  system,  is 
also  equivalent  to  immense  quantities  of  heat.  If  our  earth 
were  by  a  sudden  shock  brought  to  rest  on  her  orbit — which 
is  not  to  be  feared  in  the  existing  arrangements  of  our  system 
— by  such  a  shock  a  quantity  of  heat  would  be  generated 
equal  to  that  produced  by  the  combustion  of  fourteen  such 
earths  of  solid  coal.  Making  the  most  unfavourable  assump- 
tion as  to  its  capacity  for  heat,  that  is,  placing  it  equal  to  that 
of  water,  the  mass  of  the  earth  would  thereby  be  heated  11,200 
degrees  ;  it  would  therefore  be  quite  fused  and  for  the  most 
part  reduced  to  vapour.  If,  then,  the  earth,  after  having  been 
thus  brought  to  rest,  should  fall  into  the  sun,  which  of  course 
would  be  the  case,  the  quantity  of  heat  developed  by  the  shock 
would  be  four  hundred  times  greater. 

Even  now,  from  time  to  time,  such  a  process  is  repeated 
on  a  small  scale.  There  can  hardly  be  a  doubt  that  meteors, 
fire-balls,  and  meteoric  stones,  are  masses  which  belong  to 
the  universe,  and  before  coming  into  the  domain  of  our  earth, 
moved  like  the  planets  round  the  sun.  Only  when  they  enter 
our  atmosphere  do  they  become  visible  and  fall  sometimes  to 
the  earth.  In  order  to  explain  the  emission  of  light  by 
these  bodies,  and  the  fact  that  for  some  time  after  their 
descent  they  are  very  hot,  the  friction  was  long  ago  thought 
of  which  they  experience  in  passing  through  the  air.  We 
can  now  calculate  that  a  velocity  of  3000  feet  a  second, 


THE  LIGHT  AND   HEAT  OF   METEORS.  235 

supposing  the  whole  of  the  friction  to  be  expended  in  heating 
the  solid  mass,  would  raise  a  piece  of  meteoric  iron  1000°  C. 
in  temperature,  or,  in  other  words,  to  a  vivid  red  heat.  Now 
the  average  velocity  of  the  meteors  seems  to  be  thirty  or  forty 
times  the  above  amount.  To  compensate  this,  however,  the 
greater  portion  of  the  heat  is,  doubtless,  carried  away  by  the 
condensed  mass  of  air  which  the  meteor  drives  before  it.  It 
is  known  that  bright  meteors  generally  leave  a  luminous  trail 
behind  them,  which  probably  consists  of  several  portions  of 
the  red-hot  surfaces.  Meteoric  masses  which  fall  to  the  earth 
often  burst  with  a  violent  explosion,  which  may  be  regarded 
as  a  result  of  the  quick  heating.  The  newly-fallen  pieces 
have  been  for  the  most  part  found  hot,  but  not  red-hot,  which 
is  easily  explainable  by  the  circumstance,  that  during  the 
short  time  occupied  by  the  meteor  in  passing  through  the 
atmosphere,  only  a  thin,  superficial  layer  is  heated  to  redness, 
while  but  a  small  quantity  of  heat  has  been  able  to  penetrate 
to  the  interior  of  the  mass.  For  this  reason  the  red  heat  can 
speedily  disappear. 

Thus  has  the  falling  of  the  meteoric  stone,  the  minute 
remnant  of  processes  which  seems  to  have  played  an  impor- 
tant part  in  the  formation  of  the  heavenly  bodies,  conducted 
us  to  the  present  time,  where  we  pass  from  the  darkness  of 
hypothetical  views  to  the  brightness  of  knowledge.  In  what 
we  have  said,  however,  all  that  is  hypothetical  is  the  assump- 
tion of  Kant  and  Laplace,  that  the  masses  of  our  system  were 
once  distributed  as  nebulae  in  space. 

On  account  of  the  rarity  of  the  case,  we  will  still  further 
remark,  in  what  close  coincidence  the  results  of  science  here 
stand  with  the  earlier  legends  of  the  human  family,  and  the 
forebodings  of  poetic  fancy.  The  cosmogony  of  ancient  na- 
tions generally  commences  with  chaos  and  darkness. 

Neither  is  the  Mosaic  tradition  very  divergent,  particu- 
larly when  we  remember  that  that  which  Moses  names  heaven 
is  different  from  the  blue  dome  above  us,  and  is  synonymous 


236  INTERACTION  OF  NATURAL  FOECES. 

with  space,  and  that  the  unformed  earth,  and  the  waters  of 
the  great  deep,  which  were  afterwards  divided  into  waters 
above  the  firmament,  and  waters  below  the  firmament,  resem- 
bled the  chaotic  components  of  the  world. 

Our  earth  bears  still  the  unmistakable  traces  of  its  old 
fiery  fluid  condition.  The  granite  formations  of  her  moun- 
tains exhibit  a  structure,  which  can  only  be  produced  by  the 
crystallization  of  fused  masses.  Investigation  still  shows 
that  the  temperature  in  mines,  and  borings,  increases  as  we 
descend ;  and  if  this  increase  is  uniform,  at  the  depth  of  fifty 
miles,  a  heat  exists  sufficient  to  fuse  all  our  minerals.  Even 
now  our  volcanoes  project,  from  time  to  time,  mighty  masses 
of  fused  rocks  from  their  interior,  as  a  testimony  of  the  heat 
which  exists  there.  But  the  cooled  crust  of  the  earth  has 
already  become  so  thick,  that,  as  may  be  shown  by  calcula- 
tions of  its  conductive  power,  the  heat  coming  to  the  surface 
from  within,  in  comparison  with  that  reaching  the  earth  from 
the  sun,  is  exceedingly  small,  and  increases  the  temperature 
of  the  surface  only  about  one  thirtieth  of  a  degree  centigrade  ; 
so  that  the  remnant  of  the  old  store  of  force  which  is  enclosed 
as  heat  within  the  bowels  of  the  earth,  has  a  sensible  influence 
upon  the  processes  at  the  earth's  surface,  only  through  the 
instrumentality  of  volcanic  phenomena.  These  processes  owe 
their  power  almost  wholly  to  the  action  of  other  heavenly  bodies, 
particularly  to  the  light  and  heat  of  the  sun,  and  partly  also, 
in  the  case  of  the  tides^  to  the  attraction  of  the  sun  and  moon. 

Most  varied  and  numerous  are  the  changes  which  we  owe 
to  the  light  and  heat  of  the  sun.  The  sun  heats  our  atmos- 
phere irregularly,  the  warm  rarefied  air  ascends,  while  fresh 
cool  air  flows  from  the  sides  to  supply  its  place  :  in  this  way 
winds  are  generated.  This  action  is  most  powerful  at  the 
equator,  the  warm  air  of  which  incessantly  flows  in  the  upper 
regions  of  the  atmosphere  towards  the  poles :  while  just  as 
persistently,  at  the  earth's  surface,  the  trade  wind  carries  new 
and  cool  air  to  the  equator.  Without  the  heat  of  the  sun  all 


FORCE  PRODUCES  THE  WATER  CIRCULATIONS.    237 

winds  must,  of  necessity,  cease.  Similar  currents  are  pro- 
duced by  the  same  cause  in  the  waters  of  the  sea.  Their  pow- 
er may  be  inferred  from  the  influence  which  in  some  cases 
they  exert  upon  climate.  By  them  the  warm  water  of  the 
Antilles  is  carried  to  the  British  Isles,  and  confers  upon  them 
a  mild,  uniform  warmth  and  rich  moisture ;  while,  through 
similar  causes,  the  floating  ice  of  the  North  Pole  is  carried  to 
the  coast  of  Newfoundland,  and  produces  cold.  Further,  by 
the  heat  of  the  sun,  a  portion  of  the  water  is  converted  into 
vapour  which  rises  in  the  atmosphere,  is  condensed  to  clouds, 
or  falls  in  rain  and  snow  upon  the  earth,  collects  in  the  form 
of  springs,  brooks,  and  rivers,  and  finally  reaches  the  sea 
again,  after  having  gnawed  the  rocks,  carried  away  the  light 
earth,  and  thus  performed  its  part  in  the  geologic  changes  of 
the  earth ;  perhaps,  besides  all  this  it  has  driven  our  water- 
mill  upon  its  way.  If  the  heat  of  the  sun  were  withdrawn, 
there  would  remain  only  a  single  motion  of  water,  namely, 
the  tides,  which  are  produced  by  the  attraction  of  the  sun  and 
moon. 

How  is  it,  now,  with  the  motions  and  the  work  of  organic 
beings.  To  the  builders  of  the  automata  of  the  last  century, 
men  and  animals  appeared  as  clockwork  which  was  never 
wound  up,  and  created  the  force  which  they  exerted  out  of 
nothing.  They  did  not  know  how  to  establish  a  connection 
between  the  nutriment  consumed  and  the  work  generated. 
Since,  however,  we  have  learned  to  discern  in  the  steam-en- 
gine this  origin  of  mechanical  force,  we  must  inquire  whether 
something  similar  does  not  hold  good  with  regard  to  men.  In- 
deed, the  continuation  of  life  is  dependent  on  the  consumption 
of  nutritive  materials  :  these  are  combustible  substances,  which, 
after  digestion  and  being  passed  into  the  blood,  actually  under- 
go a  slow  combustion,  and  finally  enter  into  almost  the  same 
combinations  with  the  oxygen  of  the  atmosphere  that  are  pro- 
duced in  an  open  fire.  As  the  Quantity  of  heat  generated  by 
combustion  is  independent  of  the  duration  of  the  combustion  and 


238        INTEKACTION  OF  NATUBAL  FOKCE8. 

the  steps  in  which  it  occurs,  we  can  calculate  from  the  mass  of 
the  consumed  material  how  much  heat,  or  its  equivalent  work 
is  thereby  generated  in  an  animal  body.  Unfortunately,  the 
difficulty  of  the  experiments  is  still  very  great ;  but  within 
those  limits  of  accuracy  which  have  been  as  yet  attainable,  the 
experiments  show  that  the  heat  generated  in  the  animal  body 
corresponds  to  the  amount  which  would  be  generated  by  the 
chemical  processes.  The  animal  body  therefore  does  not  differ 
from  the  steam-engine,  as  regards  the  manner  in  which  it 
obtains  heat  and  force,  but  does  differ  from  it  in  the  man- 
ner in  which  the  force  gained  is  to  be  made  use  of.  The 
body  is,  besides,  more  limited  than  the  machine  in  the  choice 
of  its  fuel ;  the  latter  could  be  heated  with  sugar,  with  starch- 
flour,  and  butter,  just  as  well  as  with  coal  or  wood ;  the  ani- 
mal body  must  dissolve  its  materials  artificially,  and  distribute 
them  through  its  system ;  it  must,  further,  perpetually  renew 
the  used-up  materials  of  its  organs,  and  as  it  cannot  itself 
create  the  matter  necessary  for  this,  the  matter  must  come 
from  without.  Liebig  was  the  first  to  point  out  these  various 
uses  of  the  consumed  nutriment.  As  material  for  the  perpet- 
ual renewal  of  the  body,  it  seems  that  certain  definite  albumi- 
nous substances  which  appear  in  plants,  and  form  the  chief 
mass  of  the  animal  body,  can  alone  be  used.  They  form  only 
a  portion  of  the  mass  of  nutriment  taken  daily ;  the  remain- 
der, sugar,  starch,  fat,  are  really  only  materials  for  warming, 
and  are  perhaps  not  to  be  superseded  by  coal,  simply  because 
the  latter  does  not  permit  itself  to  be  dissolved. 

If,  then,  the  processes  in  the  animal  body  are  not  in  this 
respect  to  be  distinguished  from  inorganic  processes,  the  ques- 
tion arises,  whence  comes  the  nutriment  which  constitutes 
the  source  of  the  body's  force  ?  The  answer  is,  from  the 
vegetable  kingdom ;  for  only  the  material  of  plants,  or  the 
flesh  of  plant-eating  animals,  can  be  made  use  of  for  food. 
The  animals  which  live  on  plants  occupy  a  mean  position 
between  carnivorous  animals,  in  which  we  reckon  man,  and 


SOLAR  ORIGIN   OF   ORGANIC  FORCE.  239 

vegetables,  which  the  former  could  not  make  use  of  immedi- 
ately as  nutriment.  In  hay  and  grass  the  same  nutritive  sub- 
stances are  present  as  in  meal  and  flour,  but  in  less  quantity. 
As,  however,  the  digestive  organs  of  man  are  not  in  a  condi- 
tion to  extract  the  small  quantity  of  the  useful  from  the  great 
excess  of  the  insoluble,  we  submit,  in  the  first  place,  these 
substances  to  the  powerful  digestion  of  the  ox,  permit  the 
nourishment  to  store  itself  in  the  animal's  body,  in  order  in 
the  end  to  gain  it  for  ourselves  in  a  more  agreeable  and  use- 
ful form.  In  answer  to  our  question,  therefore,  we  are  re- 
ferred to  the  vegetable  world.  Now  when  what  plants  take 
in  and  what  they  give  out  are  made  the  subjects  of  investiga- 
tion, we  find  that  the  principal  part  of  the  former  consists  in 
the  products  of  combustion  which  are  generated  by  the  ani- 
mal. They  take  the  consumed  carbon  given  on0  in  respira- 
tion, as  carbonic  acid,  from  the  air,  the  consumed  hydrogen 
as  water,  the  nitrogen  in  its  simplest  and  closest  combination 
as  ammonia ;  and  from  these  materials,  with  the  assistance 
of  small  ingredients  which  they  take  from  the  soil,  they  gen- 
erate anew  the  compound  combustible  substances,  albumen, 
sugar,  oil,  on  which  the  animal  subsists.  Here,  therefore,  is 
a  circuit  which  appears  to  be  a  perpetual  store  of  force. 
Plants  prepare  fuel  and  nutriment,  animals  consume  these, 
burn  them  slowly  in  their  lungs,  and  from  the  products  of 
combustion  the  plants  again  derive  their  nutriment.  The 
latter  is  an  eternal  source  of  chemical,  the  former  of  mechan- 
ical forces.  Would  not  the  combination  of  both  organic  king- 
doms produce  the  perpetual  motion  ?  We  must  not  conclude 
hastily :  further  inquiry  shows,  that  plants  are  capable  of  pro- 
ducing combustible  substances  only  when  they  are  under  the 
influence  of  the  sun.  A  portion  of  the  sun's  rays  exhibits  a 
remarkable  relation  to  chemical  forces, — it  can  produce  and 
destroy  chemical  combinations  ;  and  these  rays,  which  for  the 
most  part  are  blue  or  violet,  are  called  therefore  chemical 
rays.  We  make  use  of  their  action  in  the  production  of  pho- 


240  INTERACTION  OF  NATURAL  FORCES. 

tographs.  Here  compounds  of  silver  are  decomposed  at  the 
place  where  the  sun's  rays  strike  them.  The  same  rays  over- 
power in  the  green  leaves  of  plants  the  strong  chemical  affinity 
of  the  carbon  of  the  carbonic  acid  for  oxygen,  give  back  the 
latter  free  to  the  atmosphere,  and  accumulate  the  other,  in 
combination  with  other  bodies,  as  woody  fibre,  starch,  oil,  or 
resin.  These  chemically  active  rays  of  the  sun  disappear 
completely  as  soon  as  they  encounter  the  green  portions  of  the 
plants,  and  hence  it  is  that  in  daguerrotype  images  the  green 
leaves  of  plants  appear  uniformly  black.  Inasmuch  as  the 
light  coming  from  them  does  not  contain  the  chemical  rays,  it 
is  unable  to  act  upon  the  silver  compounds. 

Hence  a  certain  portion  of  force  disappears  from  the  sun- 
light, while  combustible  substances  are  generated  and  accumu- 
lated in  plants  ;  and  we  can  assume  it  as  very  probable,  that 
the  former  is  the  cause  of  the  latter.  I  must  indeed  remark, 
that  we  are  in  possession  of  no  experiments  from  which  we 
might  determine  whether  the  vis  viva  of  the  sun's  rays  which 
have  disappeared,  corresponds  to  the  chemical  forces  accumu- 
lated during  the  same  time  ;  and  as  long  as  these  experiments 
are  wanting,  we  cannot  regard  the  stated  relation  as  a  cer- 
tainty. If  this  view  should  prove  correct,  we  derive  from  it 
the  flattering  result,  that  all  force,  by  means  of  which  our  bodies 
live  and  move,  finds  its  source  in  the  purest  sunlight ;  and 
hence  we  are  all,  in  point  of  nobility,  not  behind  the  race  of  the 
great  monarch  of  China,  who  heretofore  alone  called  himself 
Son  of  the  Sun.  But  it  must  also  be  conceded  that  our  lower 
fellow-beings,  the  frog  and  leech,  share  the  same  ethereal 
origin,  as  also  the  whole  vegetable  world,  and  even  the  fuel 
which  comes  to  us  from  the  ages  past,  as  well  as  the  youngest 
offspring  of  the  forest  with  which  we  heat  our  stoves  and  set 
our  machines  in  motion. 

You  see,  then,  that  the  immense  wealth  of  ever-changing 
meteorological,  climatic,  geological,  and  organic  processes  of 
our  earth  are  almost  wholly  preserved  in  action  by  the  light 


DYNAMICS  OF   SUNLIGHT.  241 

and  heat-giving  rays  of  the  sun ;  and  you  see  in  this  a  re- 
markable example,  how  Proteus-like  the  effects  of  a  single 
cause,  under  altered  external  conditions,  may  exhibit  itself  in 
nature.  Besides  these,  the  earth  experiences  an  action  of 
another  kind  from  its  central  luminary,  as  well  as  from  its 
satellite  the  moon,  which  exhibits  itself  in  the  remarkable 
phenomenon  of  the  ebb  and  flow  of  the  tide. 

Each  of  these  bodies  excites,  by  its  attraction  upon  the 
waters  of  the  sea,  two  gigantic  waves,  which  flow  in  the  same 
direction  round  the  world,  as  the  attracting  bodies  themselves 
apparently  do.  The  two  waves  of  the  moon,  on  account  of 
her  greater  nearness,  are  about  three  and  a  half  times  as  large 
as  those  excited  by  the  sun.  One  of  these  waves  has  its  crest 
on  the  quarter  of  the  earth's  surface  which  is  turned  towards 
the  moon,  the  other  is  at  the  opposite  side.  Both  these  quar- 
ters possess  the  flow  of  the  tide,  while  the  regions  which  lie 
between  have  the  ebb.  Although  in  the  open  sea  the  height 
of  the  tide  amounts  to  only  about  three  feet,  and  only  in  cer- 
tain narrow  channels,  where  the  moving  water  is  squeezed 
together,  rises  to  thirty  feet,  the  might  of  the  phenomena  is 
nevertheless  manifest  from  the  calculation  of  Bessel,  accord- 
ing to  which  a  quarter  of  the  earth  covered  by  the  sea  pos- 
sesses, during  the  flow  of  the  tide,  about  25,000  cubic  miles 
of  water  more  than  during  the  ebb,  and  that  therefore  such  a 
mass  of  water  must,  in  six  and  a  quarter  hours,  flow  from 
one  quarter  of  the  earth  to  the  other. 

The  phenomena  of  the  ebb  and  flow,  as  already  recognized 
by  Mayer,  combined  with  the  law  of  the  conservation  of  force, 
stand  in  remarkable  connection  with  the  question  of  the  sta- 
bility of  our  planetary  system.  The  mechanical  theory  of  the 
planetary  motions  discovered  by  Newton  teaches,  that  if  a 
solid  body  in  absolute  vacuo,  attracted  by  the  sun,  move 
around  him  in  the  same  manner  as  the  planets,  this  motion 
will  endure  unchanged  through  all  eternity. 

Now  we  have  actually  not  only  one,  but  several  such 
11 


24:2  INTERACTION  OF  NATURAL  FORCES. 

planets,  which  move  around  the  sun,  and  by  their  mutual 
attraction  create  little  changes  and  disturbances  in  each  other's 
paths.  Nevertheless  Laplace,  in  his  great  work,  the  Mecan- 
ique  Celeste,  has  proved  that  in  our  planetary  system  all  these 
disturbances  increase  and  diminish  periodically,  and  can  never 
exceed  certain  limits,  so  that  by  this  cause  the  eternal  exist- 
ence of  the  planetary  system  is  unendangered. 

But  I  have  already  named  two  assumptions  which  must  be 
made  :  first  that  the  celestial  spaces  must  be  absolutely  empty ; 
and  secondly,  that  the  sun  and  planets  must  be  solid  bodies. 
The  first  is  at  least  the  case  as  far  as  astronomical  observa- 
tions reach,  for  they  have  never  been  able  to  detect  any  retar- 
dation of  the  planets,  such  as  would  occur  if  they  moved  in  a 
resisting  medium.  But  on  a  body  of  less  mass,  the  comet  of 
Encke,  changes  are  observed  of  such  a  nature :  this  comet  de- 
scribes ellipses  round  the  sun  which  are  becoming  gradually 
smaller.  If  this  kind  of  motion,  which  certainly  corresponds 
to  that  through  a  resisting  medium,  be  actually  due  to  the  ex- 
istence of  such  a  medium,  a  time  will  come  when  the  comet 
will  strike  the  sun ;  and  a  similar  end  threatens  all  the  planets, 
although  after  a  time,  the  length  of  which  baffles  our  imagina- 
tion to  conceive  of  it.  But  even  should  the  existence  of  a  re- 
sisting medium  appear  doubtful  to  us,  there  is  no  doubt  that 
the  planets  are  not  wholly  composed  of  solid  materials  which 
are  inseparably  bound  together.  Signs  of  the  existence  of  an 
atmosphere  are  observed  on  the  Sun,  on  Venus,  Mars,  Jupi- 
ter, and  Saturn.  Signs  of  water  and  ice  upon  Mars ;  and 
our  earth  has  undoubtedly  a  fluid  portion  on  its  surface, 
and  perhaps  a  still  greater  portion  of  fluid  within  it.  The 
motions  of  the  tides,  however,  produce  friction,  all  friction 
destroys  vis  viva,  and  the  loss  in  this  case  can  only  affect  the 
vis  viva  of  the  planetary  system.  We  come  thereby  to  the 
unavoidable  conclusion,  that  every  tide,  although  with  infinite 
slowness,  still  with  certainty,  diminishes  the  store  of  mechani- 
cal force  of  the  system ;  and  as  a  consequence  of  this,  the  ro- 


tation  of  the  planets  in  question  round  their  axes  must  become 
more  slow ;  they  must  therefore  approach  the  sun,  or  their 
satellites  must  approach  them.  What  length  of  time  must 
pass  before  the  length  of  our  day  is  diminished  one  second  by 
the  action  of  the  tide  cannot  be  calculated,  until  the  height 
and  time  of  the  tide  in  all  portions  of  the  ocean  are  known. 
This  alteration,  however,  takes  place  with  extreme  slowness, 
as  is  known  by  the  consequences  which  Laplace  has  deduced 
from  the  observations  of  Hipparchus,  according  to  which, 
during  a  period  of  2000  years,  the  duration  of  the  day  has 
not  been  shortened  by  the  one  three  hundredth  part  of  a  sec- 
ond. The  final  consequence  would  be,  but  after  millions  of 
years,  if  in  the  mean  time  the  ocean  did  not  become  frozen, 
that  one  side  of  the  earth  would  be  constantly  turned  towards 
the  sun,  and  enjoy  a  perpetual  day,  whereas  the  opposite  side 
would  be  involved  in  eternal  night.  Such  a  position  we 
observe  in  our  moon  with  regard  to  the  earth,  and  also  in  the 
case  of  the  satellites  as  regards  their  planets  ;  it  is,  perhaps, 
due  to  the  action  of  the  mighty  ebb  and  flow  to  which  these 
bodies,  in  the  time  of  their  fiery  fluid  condition,  were  sub- 
jected. 

I  would  not  have  brought  forward  these  conclusions,  which 
again  plunge  us  in  the  most  distant  future,  if  they  were  not 
unavoidable.  Physico-mechanical  laws  are,  as  it  were,  the 
telescopes  of  our  spiritual  eye,  which  can  penetrate  into  the 
deepest  night  of  time,  past  and  to  come. 

Another  essential  question  as  regards  the  future  of  our 
planetary  system  has  reference  to  its  future  temperature  and 
illumination.  As  the  internal  heat  of  the  earth  has  but  little 
influence  on  the  temperature  of  the  surface,  the  heat  of  the 
sun  is  the  only  thing  which  essentially  affects  the  question. 
The  quantity  of  heat  falling  from  the  sun  during  a  given  time 
upon  a  given  portion  of  the  earth's  surface  may  be  measured, 
and  from  this  it  can  be  calculated  how  much  heat  in  a  given 
time  is  sent  out  from  the  entire  sun.  Such  measurements 


244  INTEKACTION   OF  NATUEAL   FOKCES. 

have  been  made  by  the  French  physicist  Pouillet,  and  it  has 
been  found  that  the  sun  gives  out  a  quantity  of  heat  per  hour 
equal  to  that  which  a  layer  of  the  densest  coal  ten  feet  thick 
would  give  out  by  its  combustion ;  and  hence  in  a  year  a 
quantity  equal  to  the  combustion  of  a  layer  of  seventeen  miles. 
If  this  heat  were  drawn  uniformly  from  the  entire  mass  of  the 
sun,  its  temperature  would  only  be  diminished  thereby  one  and 
one  third  of  a  degree  centigrade  per  year,  assuming  its  capa- 
city for  heat  to  be  equal  to  that  of  water.  These  results  can 
give  us  an  idea  of  the  magnitude  of  the  emission,  in  relation 
to  the  surface  and  mass  of  the  sun  ;  but  they  cannot  inform 
us  whether  the  sun  radiates  heat  as  a  glowing  body,  which 
since  its  formation  has  its  heat  accumulated  within  it,  or 
whether  a  new  generation  of  heat  by  chemical  processes 
takes  place  at  the  sun's  surface.  At  all  events  the  law  of  the 
conservation  of  force  teaches  us  that  no  process  analogous  to 
those  known  at  the  surface  of  the  earth,  can  supply  for  eternity 
an  inexhaustible  amount  of  light  and  heat  to  the  sun.  But 
the  same  law  also  teaches  that  the  store  of  force  at  present 
existing,  as  heat,  or  as  what  may  become  heat,  is  sufficient  for 
an  immeasurable  time.  With  regard  to  the  store  of  chemical 
force  in  the  sun,  we  can  form  no  conjecture,  and  the  store  of 
heat  there  existing  can  only  be  determined  by  very  uncertain 
estimations.  If,  however,  we  adopt  the  very  probable  view, 
that  the  remarkably  small  density  of  so  large  a  body  is  caused 
by  its  high  temperature,  and  may  become  greater  in  time,  it 
may  be  calculated  that  if  the  diameter  of  the  sun  were  dimin- 
ished only  the  ten-thousandth  part  of  its  present  length,  by 
this  act  a  sufficient  quantity  of  heat  would  be  generated  to 
cover  the  total  emission  for  2100  years.  Such  a  small  change 
besides  it  would  be  difficult  to  detect  even  by  the  finest  astro- 
nomical observations. 

Indeed,  from  the  commencement  of  the  period  during 
which  we  possess  historic  accounts,  that  is,  for  a  period  of 
about  4000  years,  the  temperature  of  the  earth  has  not  sensi- 


CONSTANCY   OF  THE  EARTH'S   TEMPERATURE.         24:5 

bly  diminished.  From  these  old  ages  we  have  certainly  no 
thermometric  observations,  but  we  have  information  regard- 
ing the  distribution  of  certain  cultivated  plants,  the  vine,  the 
olive  tree,  which  are  very  sensitive  to  changes  of  the  mean 
annual  temperature,  and  we  find  that  these  plants  at  the  pres- 
ent moment  have  the  same  limits  of  distribution  that  they  had 
in  the  times  of  Abraham  and  Homer  ;  from  which  we  may  in- 
fer backwards  the  constancy  of  the  climate. 

In  opposition  to  this  it  has  been  urged,  that  here  in  Prussia 
the  German  knights  in  former  times  cultivated  the  vine, 
cellared  their  own  wine  and  drank  it,  which  is  no  longer  pos- 
sible. From  this  the  conclusion  has  been  drawn,  that  the 
heat  of  our  climate  has  diminished  since  the  time  referred  to. 
Against  this,  however,  Dove  has  cited  the  reports  of  ancient 
chroniclers,  according  to  which,  in  some  peculiarly  hot  years, 
the  Prussian  grape  possessed  somewhat  less  than  its  usual 
quantity  of  acid.  The  fact  also  speaks  not  so  much  for  the 
climate  of  the  country  as  for  the  throats  of  the  German 
drinkers. 

But  even  though  the  force  store  of  our  planetary  system 
is  so  immensely  great,  that  by  the  incessant  emission  which 
has  occurred  during  the  period  of  human  history  it  has  not 
been  sensibly  diminished,  even  though  the  length  of  the  time 
which  must  flow  by,  before  a  sensible  change  in  the  state  of 
our  planetary  system  occurs,  is  totally  incapable  of  measure- 
ment, still  the  inexorable  laws  of  mechanics  indicate  that  this 
store  of  force,  which  can  only  suffer  loss  and  not  gain,  must 
be  finally  exhausted.  Shall  we  terrify  ourselves  by  this 
thought  ?  Men  are  in  the  habit  of  measuring  the  greatness 
and  the  wisdom  of  the  universe  by  the  duration  and  the  profit 
which  it  promises  to  their  own  race  ;  but  the  past  history  of 
the  earth  already  shows  what  an  insignificant  moment  the 
duration  of  the  existence  of  our  race  upon  it  constitutes.  A 
Nineveh  vessel,  a  Roman  sword  awakes  in  us  the  conception 
of  grey  antiquity.  What  the  museums  of  Europe  show  us  of 


246  INTERACTION   OF  NATURAL  FORCES. 

the  remains  of  Egypt  and  Assyria  we  gaze  upon  with  silent 
astonishment,  and  despair  of  being  able  to  carry  our  thoughts 
back  to  a  period  so  remote.  Still  must  the  human  race  have 
existed  for  ages,  and  multiplied  itself  before  the  pyramids  of 
Nineveh  could  have  been  erected.  We  estimate  the  duration 
of  human  history  at  6000  years  ;  but  immeasurable  as  this  time 
may  appear  to  us,  what  is  it  in  comparison  with  the  time  dur- 
ing which  the  earth  carried  successive  series  of  rank  plants 
and  mighty  animals,  and  no  men ;  during  which  in  our  neigh- 
bourhood the  amber-tree  bloomed,  and  dropped  its  costly  gum 
on  the  earth  and  in  the  sea ;  when  in  Siberia,  Europe  and 
North  America  groves  of  tropical  palms  flourished ;  where 
gigantic  lizards,  and  after  them  elephants,  whose  mighty  re- 
mains we  still  find  buried  in  the  earth,  found  a  home  ?  Dif- 
ferent geologists,  proceeding  from  different  premises,  have 
sought  to  estimate  the  duration  of  the  above  creative  period, 
and  vary  from  a  million  to  nine  million  years.  And  the  tune 
during  which  the  earth  generated  organic  beings  is  again 
small  when  we  compare  it  with  the  ages  during  which  the 
world  was  a  ball  of  fused  rocks.  For  the  duration  of  its  cool- 
ing from  2000°  to  200°  centigrade,  the  experiments  of  Bishop 
upon  basalt  show  that  about  350  millions  of  years  would  be 
necessary.  And  with  regard  to  the  time  during  which  the  first 
nebulous  mass  condensed  into  our  planetary  system,  our  mos* 
daring  conjectures  must  cease.  The  history  of  man,  there 
fore,  is  but  a  short  ripple  in  the  ocean  of  time.  For  a  much 
longer  series  of  years  than  that  during  which  man  has  already 
occupied  this  world,  the  existence  of  the  present  state  of  in- 
organic nature  favourable  to  the  duration  of  man  seems  to  be 
secured,  so  that  for  ourselves  and  for  long  generations  after 
us,  we  have  nothing  to  fear.  But  the  same  forces  of  air  and 
water,  and  of  the  volcanic  interior,  which  produced  former 
geological  revolutions,  and  buried  one  series  of  living  forms 
after  another,  act  still  upon  the  earth's  crust.  They  more 
probably  will  bring  about  the  last  day  of  the  human  race  than 


CULMINATION   OF  THE  ARGUMENT.  247 

those  distant  cosmical  alterations  of  which  we  have  spoken, 
and  perhaps  force  us  to  make  way  for  new  and  more  com- 
plete living  forms,  as  the  lizards  and  the  mammoth  have 
given  place  to  us  and  our  fellow-creatures  which  now  exist. 

Thus  the  thread  which  was  spun  in  darkness  by  those 
who  sought  a  perpetual  motion  has  conducted  us  to  a  univer- 
sal law  of  nature,  which  radiates  light  into  the  distant  nights 
of  the  beginning  and  of  the  end  of  the  history  of  the  universe. 
To  our  own  race  it  permits  a  long  but  not  an  endless  exist- 
ence ;  it  threatens  it  with  a  day  of  judgment,  the  dawn  of 
which  is  still  happily  obscured.  As  each  of  us  singly  must 
endure  the  thought  of  his  death,  the  race  must  endure  the 
same.  But  above  the  forms  of  life  gone  by,  the  human  race 
has  higher  moral  problems  before  it,  the  bearer  of  which  it  is, 
and  in  the  completion  of  which  it  fulfils  its  destiny. 


I. 

EEMARKS  ON 

THE  FORCES  OF  INORGANIC  NATURE. 

BY  DR.  J.  R.  MAYER. 
TRANSLATED  BY  J.  C.  FOSTER,  B.A. 


n. 
CELESTIAL  DYNAMICS. 

BY  DR.  J.  R.  MAYER. 
TRANSLATED  BY  DB.  H.  DEBUS,  F.B.S. 


ra. 

BEMABKS  ON 

THE  MECHANICAL  EQUIVALENT  OF  HEAT, 

BY  DR.  J.  R.  MAYER. 

TRANSLATED  BY  J.  0.  FOSTEK,  B.A. 


11* 


JULIUS  ROBERT  MAYER  was  born  at  Heilbronn,  November  25,  1814.  He 
received  a  medical  education,  and  became  first,  county  wound-physician  and 
afterwards  city  physician  of  Heilbronn.  But  few  particulars  of  his  life  have 
been  obtained.  In  1840  he  made  a  voyage  on  a  Dutch  freighter  to  Java, 
and  it  was  the  accident  of  bleeding  a  feverish  patient  in  this  country,  and 
observing  that  the  venous  blood  in  the  tropics  was  of  a  much  brighter  red 
than  in,  colder  latitudes,  that  led  him  to  those  investigations  of  natural 
forces,  the  chief  results  of  which  are  given  in  the  following  essays.  Two 
years  after  his  attention  was  drawn  to  the  subject — in  1842,  he  published 
his  first  paper  on  the  "  Forces  of  Inorganic  Nature."  It  was  put  together 
briefly,  and  published  in  Liebig's  journal  to  secure  the  public  recognition  of 
.  his  claims.  His  second  publication,  "  On  Organic  Motion  and  Nutrition  " 
(1845),  an  able  essay  of  one  hundred  and  twelve  pages,  is  not  yet  translated. 
His  third  paper,  on  "Celestial  Dynamics,"  was  published  in  1848  ;  and  his 
fourth,  on  the  "  Mechanical  Equivalent  of  Heat,"  appeared  hi  1851. 

These  vast  and  rapid  labors  .were  too  much  for  his  strength.  His  over- 
tasked mind  gave  way,  and  he  was  taken  to  an  insane  asylum.  He,  how- 
ever, fortunately  recovered,  and  is  now  reported  as  occupied  with  the  culti- 
vation of  the  vine  hi  Heilbronn. 


I. 

THE  FORCES  OF  INORGANIC  NATURE. 


THE  following  pages  are  designed  as  an  attempt  to  an- 
swer the  questions,  What  are  we  to  understand  by 
"Forces"  ?  and  how  are  different  forces  related  to  each  other? 
Whereas  the  term  matter  implies  the  possession,  by  the  object 
to  which  it  is  applied,  of  very  definite  properties,  such  as 
weight  and  extension ;  the  term  force  conveys  for  the  most 
part  the  idea  of  something  unknown,  unsearchable,  and  hypo- 
thetical. An  attempt  to  render  the  notion  of  force  equally 
exact  with  that  of  matter,  and  so  to  denote  by  it  only  objects 
of  actual  investigation,  is  one  which,  with  the  consequences 
that  flow  from  it,  ought  not  to  be  unwelcome  to  those  who 
desire  that  their  views  of  nature  may  be  clear  and  unencum- 
bered by  hypotheses. 

Forces  are  causes :  accordingly,  we  may  in  relation  to 
them  make  full  application  of  the  principle — causa  cequat  ef- 
fectum.  If  the  cause  c  has  the  effect  e,  then  c— e;  if,  in  its 
turn,  e  is  the  cause  of  a  second  effect/,  we  have  e=f,  and  so 
on:  c=e=f...=c.  In  a  chain  of  causes  and  effects,  a 
term  or  a  part  of  a  term  can  never,  as  plainly  appears  from 
the  nature  of  an  equation,  become  equal  to  nothing.  This 
first  property  of  all  causes  we  call  their  indestructibility. 

If  the  given  cause  c  has  produced  an  effect  e  equal  to  it- 
self, it  has  in  that  very  act  ceased  to  be :  c  has  become  e  ;  if, 
after  the  production  of  e,  c  still  remained  in  whole  or  in  part, 


252  THE  FOECES   OF   INORGANIC   NATURE. 

there  must  be  still  further  effects  corresponding  to  this  re- 
maining cause  :  the  total  effect  of  c  would  thus  be  >  e,  which 
would  be  contrary  to  the  supposition  c=e.  Accordingly, 
since  c  becomes  e,  and  e  becomes/,  &c.,  we  must  regard  these 
various  magnitudes  as  different  forms  under  which  one  and 
the  same  object  makes  its  appearance.  This  capability  of 
assuming  various  forms  is  the  second  essential  property  of  all 
causes.  Taking  both  properties  together,  we  may  say,  causes 
are  (quantitatively)  indestructible  and  (qualitatively)  convert*- 
ible  objects. 

Two  classes  of  causes  occur  in  nature,  which,  so  far  as 
experience  goes,  never  pass  one  into  another.  The  first  class 
consists  of  such  causes  as  possess  the  properties  of  weight 
and  impenetrability ;  these  are  kinds  of  Matter :  the  other 
class  is  made  up  of  causes  which  are  wanting  in  the  proper- 
ties just  mentioned,  namely  Forces,  called  also  Impondera- 
bles, from  the  negative  property  that  has  been  indicated. 
Forces  are  therefore  indestructible ,  convertible,  imponderable 
objects. 

We  will  in  the  first  instance  take  matter,  to  afford  us  an 
example  of  causes  and  effects.  Explosive  gas,  H+O,  and 
water,  HO,  are  related  to  each  other  as  cause  and  effect, 
therefore  H + O = HO .  But  if  H + O  becomes  HO,  heat,  cat. , 
makes  its  appearance  as  well  as  water  ;  this  heat  must  like- 
wise have  a  cause,  x,  and  we  have  therefore  H+O-hic=HO 
-\-cal.  It  might,  however,  be  asked  whether  H+O  is  really 
=HO,  and  x—cal.^  and  not  perhaps  H+O =caL,  and  aj=HO, 
whence  the  above  equation  could  equally  be  deduced  ;  and  so 
in  many  other  cases.  The  phlogistic  chemists  recognized  the 
equation  between  cal.  and  x,  or  Phlogiston  as  they  called  it, 
and  in  so  doing  made  a  great  step  in  advance ;  but  they  in- 
volved themselves  again  in  a  system  of  mistakes  by  putting — x 
in  place  of  O  ;  thus,  for  instance,  they  obtained  H=HO+^- 

Chemistry,  whose  problem  it  is  to  set  forth  in  equations 
the  causal  connection  existing  between  the  different  kinds  of 


MATTER   AND  FORCE   AS   CAUSES.  253 

matter,  teaches  us  that  matter,  as  a  cause,  has  matter  for  its 
effect ;  but  we  are  equally  justified  in  saying  that  to  force  as 
cause,  corresponds  force  as  effect.  Since  c=e,  and  e=c,  it 
is  unnatural  to  call  one  term  of  an  equation  a  force,  and  the 
other  an  effect  of  force  or  phenomenon,  and  to  attach  differ- 
ent notions  to  the  expressions  Force  and  Phenomenon.  In 
brief,  then,  if  the  cause  is  matter,  the  effect  is  matyer ;  if  the 
cause  is  a  force,  the  effect  is  also  a  force. 

A  cause  which  brings  about  the  raising  of  a  weight  is  a 
force ;  its  effect  (the  raised  weight)  is,  accordingly,  equally  a 
force ;  or,  expressing  this  relation  in  a  more  general  form, 
separation  in  space  of  ponderable  objects  is  a  force  ;  since  this 
force  causes  the  fall  of  bodies,  we  call  it  falling  force.  Fall- 
ing force  and  fall,  or,  more  generally  still,  falling  force  and 
motion,  are  forces  which  are  related  to  each  other  as  cause 
and  effect — forces  which  are  convertible  one  into  the  other — 
two  different  forms  of  one  and  the  same  object.  For  exam- 
ple, a  weight  resting  on  the  ground  is  not  a  force  :  it  is  neither 
the  cause  of  motion,  nor  of  the  lifting  of  another  weight ;  it 
becomes  so,  however,  in  proportion  as  it  is  raised  above  the 
ground :  the  cause — the  distance  between  a  weight  and  the 
earth — and  the  effect — the  quantity  of  motion  produced — beai 
to  each  other,  as  we  learn  from  mechanics,  a  constant  rela- 
tion. 

Gravity  being  regarded  as  the  cause  of  the  falling  of  bod- 
ies, a  gravitating  force  is  spoken  of,  and  so  the  notions  of 
property  and  of  force  are  confounded  with  each  other :  pre- 
cisely that  which  is  the  essential  attribute  of  every  force — 
the  union  of  indestructibility  with  convertibility — is  wanting 
in  every  property :  between  a  property  and  a  force,  between 
gravity  and  motion,  it  is  therefore  impossible  to  establish  the 
equation  required  for  a  rightly-conceived  causal  relation.  If 
gravity  be  called  a  force,  a  cause  is  supposed  which  produces 
effects  without  itself  diminishing,  and  incorrect  conceptions 
of  the  causal  connections  of  things  are  thereby  fostered.  In 


254  THE  FORCES   OF   INORGANIC   NATURE. 

order  that  a  body  may  fall,  it  is  no  less  necessary  that  it 
should  be  lifted  up,  than  that  it  should  be  heavy  or  possess 
gravity ;  the  fall  of  bodies  ought  not  therefore  to  be  ascribed 
to  their  gravity  alone. 

It  is  the  problem  of  Mechanics  to  develop  the  equations 
which  subsist  between  falling  force  and  motion,  motion  and 
falling  force,  and  between  different  motions :  here  we  will  call 
to  mind  only  one  point.  The  magnitude  of  the  falling  force 
v  is  directly  proportional  (the  earth's  radius  being  assumed = 
oo  )  to  the  magnitude  of  the  mass  w,  and  the  height  d  to 
which  it  is  raised;  that  is,  v=md.  If  the  height  d=l,  to 
which  the  mass  m  is  raised,  is  transformed  into  the  final  ve- 
locity c=l  of  this- mass,  we  have  also  v=wc;  but  from  the 
known  relations  existing  between  d  and  c,  it  results  that,  for 
other  values  of  d  or  of  c,  the  measure  of  the  force  v  is  we2 ; 
accordingly  v=md=mc* :  the  law  of  the  conservation  of  vis 
viva  is  thus  found  to  be  based  on  the  general  law  of  the  inde 
structibility  of  causes. 

In  numberless  cases  we  see  motion  cease  without  having 
caused  another  motion  or  the  lifting  of  a  weight ;  but  a  force 
once  in  existence  cannot  be  annihilated,  it  can  only  change  its 
form  ;  and  the  question  therefore  arises,  What  other  forms  is 
force,  which  we  have  become  acquainted  with  as  falling  force 
and  motion,  capable  of  assuming?  Experience  alone  can 
lead  us  to  a  conclusion  on  this  point.  In  order  to  experi- 
ment with  advantage,  we  must  select  implements  which,  be- 
sides causing  a  real  cessation  of  motion,  are  as  little  as  possi- 
ble altered  by  the  objects  to  be  examined.  If,  for  example, 
we  rub  together  two  metal  plates,  we  see  motion  disappear, 
and  heat,  on  the  other  hand,  make  its  appearance,  and  we 
have  now  only  to  ask  whether  motion  is  the  cause  of  heat. 
In  order  to  come  to  a  decision  on  this  point,  we  must  discuss 
the  question  whether,  in  the  numberless  cases  in  which  the 
expenditure  of  motion  is  accompanied  by  the  appearance  of 
heat,  the  motion  has  not  some  other  effect  than  the  pro- 


EFFECTS   OF  DESTROYED  MOTION.  255 

duction  of  heat,  and  the  heat  some  other  cause  than  the 
motion. 

An  attempt  to  ascertain  the  effects  of  ceasing  motion  has 
never  yet  been  seriously  made  ;  without,  therefore,  wishing  to 
exclude  d  priori  the  hypothesis  which  it  may  be  possible  to 
set  up,  we  observe  only  that,  as  a  rule,  this  effect  cannot  be 
supposed  to  be  an  alteration  in  the  state  of  aggregation  of  the 
moved  (that  is,  rubbing,  &c.)  bodies.  If  we  assume  that  a 
certain  quantity  of  motion  v  is  expended  in  the  conversion  of 
a  rubbing  substance  m  into  n^  we  must  then  have  m-{-v=n, 
and  ^=ra+'v;  and  when  n  is  reconverted  into  m,  v  must  ap- 
pear again  in  some  form  or  other.  By  the  friction  of  two 
metallic  plates  continued  for  a  very  long  time,  we  can  grad- 
ually cause  the  cessation  of  an  immense  quantity  of  move- 
ment; but  would  it  ever  occur  to  us  to  look  for  even  the 
smallest  trace  of  the  force  which  has  disappeared  in  the  me- 
tallic dust  that  we  could  collect,  and  to  try  to  regain  it  thence  ? 
We  repeat,  the  motion  cannot  have  been  annihilated ;  and 
contrary,  or  positive  and  negative,  motions  cannot  be  regarded 
as  =O,  any  more  than  contrary  motions  can  come  out  of 
nothing,  'or  a  weight  can  raise  itself. 

Without  the  recognition  of  a  causal  connection  between 
motion  and  heat,  it  is  just  as  difficult  to  explain  the  produc- 
tion of  heat  as  it  is  to  give  any  account  of  the  motion  that 
disappears.  The  heat  cannot  be  derived  from  the  diminution 
of  the  volume  of-  the  rubbing  substances.  It  is  well  known 
that  two  pieces  of  ice  may  be  melted  by  rubbing  them  to- 
gether in  vacuo ;  but  let  any  one  try  to  convert  ice  into  water 
by  pressure,*  however  enormous.  Water  undergoes,  as  was 

*  Since  the  original  publication  of  this  paper,  Prof.  W.  Thomson  has 
shown  that  pressure  has  a  sensible  effect  in  liquefying  ice  (Conf.  Phil.  Mag. 
S.  3,  vol.  xxxvii.  p.  123);  but  the  experiments  of  Bunsen  and  of  Hopkins 
have  shown  that  the  melting-points  of  bodies  which  expand  on  becoming 
liquid  are  raised  by  pressure,  which  is  all  that  Mayer's  argument  requires. — 
G.  C.  F. 


256  THE  FORCES   OF   INORGANIC  NATURE. 

found  by  the  author,  a  rise  of  temperature  when  violently 
shaken.  The  water  so  heated  (from  12°  to  13°  C.)  has  a 
greater  bulk  after  being  shaken  than  it  had  before  ;  whence 
now  comes  this  quantity  of  heat,  which  by  repeated  shaking 
may  be  called  into  existence  in  the  same  apparatus  as  often 
as  we  please  ?  The  vibratory  hypothesis  of  heat  is  an  ap- 
proach toward  the  doctrine  of  heat  being  the  effect  of  mo- 
tion, but  it  does  not  favour  the  admission  of  this  causal  rela- 
tion in  its  full  generality ;  it  rather  lays  the  chief  stress  on 
uneasy  oscillations  (unbehagliche  Schwingungen). 

If  it  be  now  considered  as  established  that  in  many  cases 
(exceptio  confirmat  regulam)  no  other  effect  of  motion  can  be 
traced  except  heat,  and  that  no  other  cause  than  motion  can 
be  found  for  the  heat  that  is  produced,  we  prefer  the  assump- 
tion that  heat  proceeds  from  motion,  to  the  assumption  of  a 
cause  without  effect  and  of  an  effect  without  a  cause — -just  as 
the  chemist,  instead  of  allowing  oxygen  and  hydrogen  to  dis- 
appear without  further  investigation,  and  water  to  be  pro- 
duced in  some  inexplicable  manner,  establishes  a  connection 
between  oxygen  and  hydrogen  on  the  one  hand  and  water  on 
the  other. 

The  natural  connection  existing  between  falling  force,  mo- 
tion, and  heat  may  be  conceived  of  as  follows  :  We  know  that 
heat  makes  its  appearance  when  the  separate  particles  of  a 
body  approach  nearer  to  each  other ;  condensation  produces 
heat.  And  what  applies  to  the  smallest  particles  of  matter, 
and  the  smallest  intervals  between  them,  must  also  apply  to 
large  masses  and  to  measurable  distances.  The  falling  of  a 
weight  is  a  diminution  of  the  bulk  of  the  earth,  and  must 
therefore  without  doubt  be  related  to  the  quantity  of  heat 
thereby  developed  ;  this  quantity  of  heat  must  be  proportional 
to  the  greatness  of  the  weight  and  its  distance  from  the 
ground.  From  this  point  of  view  we  are  very  easily  led  to 
the  equations  between  falling  force,  motion,  and  heat,  that 
have  already  been  discussed. 


EQUIVALENCE  OF   HEAT  AND  MOTION.  257 

But  just  as  little  as  the  connection  between  falling  force 
and  motion  authorizes  the  conclusion  that  the  essence  of  fall- 
ing force  is  motion,  can  such  a  conclusion  be  adopted  in  the 
case  of  heat.  "We  are,  on  the  contrary,  rather  inclined  to 
infer  that,  before  it  can  become  heat,  motion — whether  sim- 
ple, or  vibratory  as  in  the  case  of  light  and  radiant  heat,  &c. 
— must  cease  to  exist  as  motion. 

If  falling  force  and  motion  are  equivalent  to  heat,  heat 
must  also  naturally  be  equivalent  to  motion  and  falling  force. 
Just  as  heat  appears  as  an  effect  of  the  diminution  of  bulk  and 
of  the  cessation  of  motion,  so  also  does  heat  disappear  as  a 
cause  when  its  effects  are  produced  in  the  shape  of  motion, 
expansion,  or  raising  of  weight. 

In  water-mills,  the  continual  diminution  in  bulk  which  the 
earth  undergoes,  owing  to  the  fall  of  the  water,  gives  rise  to 
motion,  which  afterwards  disappears  again,  calling  forth  un- 
ceasingly a  great  quantity  of  heat ;  and  inversely,  the  steam- 
engine  serves  to  decompose  heat  again  into  motion  or  the 
raising  of  weights.  A  locomotive  engine  with  its  train  may 
be  compared  to  a  distilling  apparatus  ;  the  heat  applied  under 
the  boiler  passes  off  as  motion,  and  this  is  deposited  again  as 
heat  at  the  axles  of  the  wheels. 

We  will  close  our  disquisition,  the  propositions  of  which 
have  resulted  as  necessary  consequences  from  the  principle 
"  causa  a3quat  effectum,"  and  which  are  in  accordance  with 
all  the  phenomena  of  Nature,  with  a  practical  deduction. 
The  solution  of  the  equations  subsisting  between  falling  force 
and  motion  requires  that  the  space  fallen  through  in  a  given 
time,  e.  g.  the  first  second,  should  be  experimentally  deter- 
mined ;  in  like  manner,  the  solution  of  the  equations  subsist- 
ing between  falling  force  and  motion  on  the  one  hand  and 
heat  on  the  other,  requires  an  answer  to  the  question,  How 
great  is  the  quantity  of  heat  which  corresponds  to  a  given 
quantity  of  motion  or  falling  force  ?  For  instance,  we  must 
ascertain  how  high  a  given  weight  requires  to  be  raised  above 


258  THE  FORCES   OF   INORGANIC  NATURE. 

the  ground  in  order  that  its  falling  force  may  be  equivalent  to 
the  raising  of  the  temperature  of  an  equal  weight  of  water 
from  0°  to  1°  C.  The  attempt  to  show  that  such  an  equa- 
tion is  the  expression  of  a  physical  truth  may  be  regarded  as 
the  substance  of  the  foregoing  remarks. 

By  applying  the  principles  that  have  been  set  forth  to  the 
relations  subsisting  between  the  temperature  and  the  volume 
of  gases,  we  find  that  the  sinking  of  a  mercury  column  by 
which  a  gas  is  compressed  is  equivalent  to  the  quantity  of 
heat  set  free  by  the  compression ;  and  hence  it  follows,  the 
ratio  between  the  capacity  for  heat  of  air  under  constant  pres- 
sure and  its  capacity  under  constant  volume  being  taken  as 
=  1*421,  that  the  warming  of  a  given  weight  of  water  from 
0°  to  1°  C.  corresponds  to  the  fall  of  an  equal  weight  from 
the  height  of  about  365  metres.*  If  we  compare  with  this 
result  the  working  of  our  best  steam-engines,  we  see  how 
small  a  part  only  of  the  heat  applied  under  the  boiler  is  really 
transformed  into  motion  or  the  raising  of  weights  ;  and  this 
may  serve  as  justification  for  the  attempts  at  the  profitable 
production  of  motion  by  some  other  method  than  the  expendi- 
ture of  the  chemical  difference  between  carbon  and  oxygen — 
more  particularly  by  the  transformation  into  motion  of  elec- 
tricity obtained  by  chemical  means. 

*  When  the  corrected  specific  heat  of  air  is  introduced  into  the  calcu- 
lation this  number  is  increased,  and  agrees  then  with  the  experimental  de- 
terminations of  Mr.  Joule. 


n. 

CELESTIAL    DYNAMICS. 


I.— INTRODUCTION. 

incandescent  and  luminous  body  diminishes  in 
temperature  and  luminosity  in  the  same  degree  as  it 
radiates  light  and  heat,  and  at  last,  provided  its  loss  be  not 
repaired  from  some  other  source  of  these  agencies,  becomes 
cold  and  non-luminous. 

For  light,  like  sound,  consists  of  vibrations  which  are 
communicated  by  the  luminous  or  sounding  body  to  a  sur- 
rounding medium.  It  is  perfectly  clear  that  a  body  can  only 
excite  such  vibrations  in  another  substance  when  its  own  par- 
ticles undergo  a  similar  movement ;  for  there  is  no  cause  for 
undulatory  motion  when  a  body  is  in  a  state  of  rest,  or  in  a 
state  of  equilibrium  with  the  medium  by  which  it  is  sur- 
rounded. If  a  bell  or  a  string  is  to  be  sounded,  an  external 
force  must  be  applied ;  and  this  is  the  cause  of  the  sound. 

If  the  vibratory  motion  of  a  string  could  take  place  with- 
out any  resistance,  it  would  vibrate  for  all  time  ;  but  in  this 
case  no  sound  could  be  produced,  because  sound  is  essentially 
the  propagation  of  motion ;  and  in  the  same  degree  as  the 


260  CELESTIAL   DYNAMICS. 

string  communicates  its  vibrations  to  the  surrounding  and  re- 
sisting medium  its  own  motion  becomes  weaker  and  weaker, 
until  at  last  it  sinks  into  a  state  of  rest. 

The  sun  has  often  and  appropriately  been  compared  to  an 
incessantly  sounding  bell.  But  by  what  means  is  the  power 
of  this  body  kept  up  in  undiminished  force  so  as  to  enable 
him  to  send  forth  his  rays  into  the  universe  in  such  a  grand 
and  magnificent  manner?  What  are  the  causes  which  coun- 
teract or  prevent  his  exhaustion,  and  thus  save  the  planetary 
system  from  darkness  and  deadly  cold  ? 

Some  endeavoured  to  approach  "  the  grand  secret,"  as 
Sir  "Win.  Herschel  calls  this  question,  by  the  assumption  that 
the  rays  of  the  sun,  being  themselves  perfectly  cold,  merely 
cause  the  "  substance"  of  heat,  supposed  to  be  contained  in 
bodies,  to  pass  from  a  state  of  rest  into  a  state  of  motion,  and 
that  in  order  to  send  forth  such  cold  rays  the  sun  need  not  be 
a  hot  body,  so  that,  in  spite  of  the  infinite  development  of 
light,  the  cooling  of  the  sun  was  a  matter  not  to  be  thought  of. 

It  is  plain  that  nothing  is  gained  by  such  an  explanation  ; 
for,  not  to  speak  of  the  hypothetical  "  substance"  of  heat, 
assumed  to  be  at  one  time  at  rest  and  at  another  time  in  mo- 
tion, now  cold  and  then  hot,  it  is  a  well-founded  fact  that  the 
sun  does  not  radiate  a  cold  phosphorescent  light,  but  a  light 
capable  of  warming  bodies  intensely ;  and  to  ascribe  such 
rays  to  a  cold  body  is  at  once  at  variance  with  reason  and 
experience. 

Of  course  such  and  similar  hypotheses  could  not  satisfy 
the  demands  of  exact  science,  and  I  will  therefore  try  to  ex- 
plain in  a  more  satisfactory  manner  than  has  been  done  up 
to  this  time  the  connexion  between  the  sun's  radiation  and  its 
effects.  In  doing  so,  I  have  to  claim  the  indulgence  of  scien- 
tific men,  who  are  acquainted  with  the  difficulties  of  my  task. 


SOURCES   OF  HEAT.  261 


II. —SOURCES    OF    HEAT. 

BEFORE  we  turn  our  attention  to  the  special  subject  of 
this  paper,  it  will  be  necessary  to  consider  the  means  by 
which  light  and  heat  are  produced.  Heat  may  be  obtained 
from  very  different  sources.  Combustion,  fermentation,  pu- 
trefaction, slaking  of  lime,  the  decomposition  of  chloride  of 
nitrogen  and  of  gun-cotton,  &c.  &c.,  are  all  of  them  sources 
of  heat.  The  electric  spark,  the  voltaic  current,  friction,  per- 
cussion, and  the  vital  processes  are  also  accompanied  by  the 
evolution  of  this  agent. 

A  general  law  of  nature,  which  knows  of  no  exception, 
is  the  following : — In  order  to  obtain  heat,  something  must 
be  expended  ;  this  something,  however  different  it  may  be  in 
other  respects,  can  always  be  referred  to  one  of  two  catego- 
ries :  either  it  consists  of  some  material  expended  in  a  chem- 
ical process,  or  of  some  sort  of  mechanical  work. 

When  substances  endowed  with  considerable  chemical  af- 
finity for  each  other  combine  chemically,  much  heat  is  devel- 
oped during  the  process.  We  shall  estimate  the  quantity  of 
heat  thus  set  free  by  the  number  of  kilogrammes  of  water 
which  it  would  heat  1°  C.  The  quantity  of  heat  necessary 
to  raise  one  kilogramme  of _  water  one  degree  is  called  a  unit 
of  heat. 

It  has  been  established  by  numerous  experiments  that  the 
combustion  of  one  kilogramme  of  dry  charcoal  in  oxygen,  so 
as  to  form  carbonic  acid,  yields  7200  units  of  heat,  which  fact 
may  be  briefly  expressed  by  saying  that  charcoal  furnishes 
7200°  degrees  of  heat. 

Superior  coal  yields  6000°,  perfectly  dry  wood  from  3300° 
to  3900°,  sulphur  2700,  and  hydrogen  34,600°  of  heat. 

According  to  experience,  the  number  of  units  of  heat  only 
depends  on  the  quantity  of  matter  which  is  consumed,  and 


262  CELESTIAL  DYNAMICS. 

not  on  the  conditions  under  which  the  burning  takes  place. 
The  same  amount  of  heat  is  given  out  whether  the  combus- 
tion proceeds  slowly  or  quickly,  in  atmospheric  air  or  in  pure 
oxygen  gas.  If  in  one  case  a  metal  be  burnt  in  air  and  the 
amount  of  heat  directly  measured,  and  in  another  instance 
the  same  quantity  of  metal  be  oxidized  in  a  galvanic  battery, 
the  heat  being  developed  in  some  other  place — say,  the  wire 
which  conducts  the  current, — in  both  of  these  experiments 
the  same  quantity  of  heat  will  be  observed. 

The  same  law  also  holds  good  for  the  production  of  heat 
by  mechanical  means.  The  amount  of  heat  obtained  is  only 
dependent  on  the  quantity  of  power  consumed,  and  is  quite 
independent  of  the  manner  in  which  this  power  has  been  ex- 
pended. If,  therefore,  the  amount  of  heat  which  is  produced 
by  certain  mechanical  work  is  known,  the  quantity  which 
will  be  obtained  by  any  other  amount  of  mechanical  work 
can  easily  be  found  by  calculation.  It  is  of  no  consequence 
whether,  this  work  consists  in  the  compression,  percussion,  or 
friction  of  bodies. 

The  amount  of  mechanical  work  done  by  a  force  may  be 
expressed  by  a  weight,  and  the  height  to  which  this  weight 
would  be  raised  by  the  same  force.  The  mathematical  ex- 
pression for  "work  done,"  that  is  to  say,  a  measure  for  this 
work,  is  obtained  by  multiplying  the  height  expressed  in  feet 
or  other  units  by  the  number  of  pounds  or  kilogrammes  lifted 
to  this  height. 

We  shall  take  one  kilogramme  as  the  unit  of  weight,  and 
one  metre  as  the  unit  of  height,  and  we  thus  obtain  the 
weight  of  one  kilogramme  raised  to  the  height  of  one  metre 
as  a  unit  measure  of  mechanical  work  performed.  This 
measure  we  shall  call  a  kilogrammetre,  and  adopt -for  it  the 
symbol  Km. 

Mechanical  work  may  likewise  be  measured  by  the  velo 
city  obtained  by  a  given  weight  in  passing  from  a  state  of  rest 
into  that  of  motion.  The  work  done  is  then  expressed  by 


SOURCES  OF  HEAT.  263 

the  product  obtained  by  the  multiplication  of  the  weight  by 
the  square  of  its  velocity.  The  first  method,  however,  be- 
cause it  is  the  more  convenient,  is  the  one  usually  adopted ; 
and  the  numbers  obtained  therefrom  may  easily  be  expressed 
in  other  units. 

The  product  resulting  from  the  multiplication  of  the  num- 
ber of  units  of  weight  and  measures  of  height,  or,  as  it  is 
called,  the  product  of  mass  and  height,  as  well  as  the  pro- 
duct of  the  mass  and  the  square  of  its  velocity,  are  called  "  vis 
viva  of  motion,"  "  mechanical  effect,"  dynamical  effect," 
"  work  done,"  "  quantite  de  travail"  &c.  &c. 

The  amount  of  mechanical  work  necessary  for  the  heating 
of  1  kilogramme  of  water  1°  C  has  been  determined  by  ex- 
periment to  be  =  367  Km ;  therefore  Km  =  0-00273  units 
of  heat.* 

A  mass  which  has  fallen  through  a  height  of  367  metres 
possesses  a  velocity  of  84-8  metres  in  one  second ;  a  mass, 
therefore,  moving  with  this  velocity  originates  1°  C.  of  heat 
when  its  motion  is  lost  by  percussion,  friction,  &c.  If  the 
velocity  be  two  or  three  times  as  great,  4°  or  9°  of  heat  will 
be  developed.  Generally  speaking,  when  the  velocity  is  c 
metres,  the  corresponding  development  of  heat  will  be  ex- 
pressed by  the  formula 

0-000139°  Xc2. 

*  This  essay  was  published  in  1845.  At  that  time  de  la  Roche  and 
Berard's  determination  of  the  specific  heat  of  air  was  generally  accepted. 
If  the  physical  constants  used  by  Mayer  be  corrected  according  to  the  re- 
sults of  more  recent  investigation,  the  mechanical  equivalent  of  heat  is 
found  to  be  771*4  foot-pounds.  Mr.  Joule  finds  it  =  772  foot-pounds.— 
TH. 


264:  CELESTIAL  DYNAMICS. 


III.— MEASURE  OF  THE  SUN'S  HEAT. 

THE  actinometer  is  an  instrument  invented  by  Sir  John 
Herschel  for  the  purpose  of  measuring  the  heating  effiect 
produced  by  the  sun's  rays.  It  is  essentially  a  thermometer 
with  a  large  cylindrical  bulb  filled  with  a  blue  liquid,  which 
is  acted  upon  by  the  sun's  rays,  and  the  expansion  of  which 
is  measured  by  a  graduated  scale. 

From  observations  made  with  this  instrument,  Sir  John 
Herschel  calculates  the  amount  of  heat  received  from  the  sun 
to  be  sufficient  to  melt  annually  at  the  surface  of  the  globe  a 
crust  of  ice  29'2  metres  in  thickness. 

Pouillet  has  recently  shown  by  some  careful  experiments 
with  the  lens  pyrheliometer,  an  instrument  invented  by  him- 
self, that  every  square  centimetre  of  the  surface  of  our  globe 
receives,  on  an  average,  in  one  minute  an  amount  of  solar 
heat  which  wo,uld  raise  the  temperature  of  one  gramme  of 
water  0'4408°.  Not  much  more  than  one-half  of  this  quan- 
tity of  heat,  however,  reaches  the  solid  surface  of  our  globe, 
since  a  considerable  portion  of  it  is  absorbed  by  our  atmo- 
sphere. The  layer  of  ice  which,  according  to  Pouillet,  could 
be  melted  by  the  solar  heat  which  yearly  reaches  our  globe 
would  have  a  thickness  of  30-89  metres. 

A  square  metre  of  our  earth's  surface  receives,  therefore, 
according  to  Pouillet's  results,  which  we  shall  adopt  in  the 
following  pages,  on  an  average  in  one  minute  4'408  units  of 
heat.  The  whole  surface  of  the  earth  is  =  9,260,500  geo- 
graphical square  miles*  ;  consequently  the  earth  receives  in 
one  minute  2247  billions  of  units  of  heat  from  the  sun. 

In  order  to  obtain  smaller  numbers,  we  shall  call  the 
quantity  of  heat  necessary  to  raise  a  cubic  mile  of  water  1° 

*  The  geographical  mile  =  7420  metres,  and  one  English  mile  =  1608 
metres. 


MEASURE   OF   THE   SUN'S   HEAT.  265 

C.  in  temperature,  a  cubic  mile  of  heat.  Since  one  cubic 
mile  of  water  weighs  408*54  billions  of  kilogrammes,  a  cubic 
mile  of  heat  contains  408*54  billions  of  units  of  heat.  The 
effect  produced  by  the  rays  of  the  sun  on  the  surface  of  the 
earth  in  one  minute  is  therefore  5*5  cubic  miles  of  heat. 

Let  us  imagine  the  sun  to  be  surrounded  by  a  hollow 
sphere  whose  radius  is  equal  to  the  mean  distance  of  the 
earth  from  the  sun,  or  20,589,000  geographical  miles ;  the 
surface  of  this  sphere  would  be  equal  to  5326  billions  of 
square  miles.  The  surface  obtained  by  the  intersection  of 
this  hollow  sphere  and  our  globe,  or  the  base  of  the  cone  of 
solar  light  which  reaches  our  earth,  stands  to  the  whole  sur- 
face of  this  hollow  sphere  as  9-^^  :  5326  billions,  or  as  1  to 
2300  millions.  This  is  the  ratio  of  the  heat  received  by 
our  globe  to  the  whole  amount  of  heat  sent  forth  from  the 
sun,  which  latter  in  one  minute  amounts  to  12,650  millions 
of  cubic  miles  of  heat. 

This  amazing  radiation  ought,  unless  the  loss  is  by  some 
means  made  good,  to  cool  considerably  even  a  body  of  the 
magnitude  of  the  sun. 

If  we  assume  the  sun  to  be  endowed  with  the  same  capa- 
city for  heat  as  a  mass  of  water  of  the  same  volume,  and  its 
loss  of  heat  by  radiation  to  affect  uniformly  its  whole  mass, 
the  temperature  of  the  sun  ought  to  decrease  1°'8  C.  yearly, 
and  for  the  historic  time  of  5000  years  this  loss  would  conse- 
quently amount  to  9000°  C. 

A  uniform  cooling  of  the  whole  of  the  sun's  huge  mass 
cannot,  however,  take  place  ;  on  the  contrary,  if  the  radiation 
were  to  occur  at  the  expense  of  a  given  store  of  heat  or  ra- 
diant power,  the  sun  would  become  covered  in  a  short  space 
of  time  with  a  cold  crust,  whereby  radiation  would  be  brought 
to  an  end.  Considering  the  continued  activity  of  the  sun 
through  countless  centuries,  we  may  assume  with  mathemati- 
cal certainty  the  existence  of  some  compensating  influence  to 
make  good  its  enormous  loss. 
12 


266  CELESTIAL    DYNAMICS. 

Is  this  restoring  agency  a  chemical  process? 

If  such  were  the  case,  the  most  favourable  assumption 
would  be  to  suppose  the  whole  mass  of  the  sun  to  be  one 
lump  of  coal,  the  combustion  of  every  kilogramme  of  which 
produces  6000  units  of  heat.  Then  the  sun  would  only  be 
able  to  sustain  for  forty-six  centuries  its  present  expenditure 
of  light  and  heat,  not  to  mention  the  oxygen  necessary  to 
keep  up  such  an  immense  combustion,  and  other  unfavourable 
circumstances. 

The  revolution  of  the  sun  on  his  axis  has  been  suggested 
as  the  cause  of  his  radiating  energy.  A  closer  examination 
proves  this  hypothesis  also  to  be  untenable. 

Rapid  rotation,  without  friction  or  resistance,  cannot  in 
itself  alone  be  regarded  as  a  cause  of  light  and  heat,  espe- 
cially as  the  sun  is  in  no  way  to  be  distinguished  from  the 
other  bodies  of  our  system  by  velocity  of  axial  rotation.  The 
sun  turns  on  his  axis  in  about  twenty-five  days,  and  his  diam- 
eter is  nearly  112  times  as  great  as  that  of  the  earth,  from 
which  it  follows  that  a  point  on  the  solar  equator  travels  but 
a  little  more  than  four  times  as  quickly  as  a  point  on  the 
earth's  equator.  The  largest  planet  of  the  solar  system, 
whose  diameter  is  about  ^th  that  of  the  sun,  turns  on  its  axis 
in  less  than  ten  hours ;  a  point  on  its  equator  revolves  about 
six  times  quicker  than  one  on  the  solar  equator.  The  outer 
ring  of  Saturn  exceeds  the  sun's  equator  more  than  ten  times 
in  velocity  of  rotation.  Nevertheless  no  generation  of  light 
or  heat  is  observed  on  our  globe,  on  Jupiter,  or  on  the  ring 
of  Saturn. 

It  might  be  thought  that  friction,  though  undeveloped  in 
the  case  of  the  other  celestial  bodies,  might  be  engendered  by 
the  sun's  rotation,  and  that  such  friction  might  generate  enor- 
mous quantities  of  heat.  But  for  the  production  of  friction 
two  bodies,  at  least,  are  always  necessary  which  are  in  imme- 
diate contact  with  one  another,  and  which  move  with  differ- 
ent velocities  or  in  different  directions.  Friction,  moreover, 


MEASURE   OF   THE   6UN?S   HEAT.  267 

has  a  tendency  to  produce  equal  motion  of  the  two  rubbing 
bodies ;  and  when  this  is  attained,  the  generation  of  heat 
ceases.  If  now  the  sun  be  the  one  moving  body,  where  is 
the  other  ?  and  if  the  second  body  exist,  what  power  prevents 
it  from  assuming  the  same  rotary  motion  as  the  sun  ? 

But  could  even  these  difficulties  be  disregarded,  a  weight- 
ier and  more  formidable  obstacle  opposes  this  hypothesis. 
The  known  volume  and  mass  of  the  sun  allow  us  to  calculate 
the  vis  viva  which  he  possesses  in  consequence  of  his  rotation. 
Assuming  his  density  to  be  uniform  throughout  his  mass,  and 
his  period  of  rotation  twenty-five  days,  it  is  equal  to  182,300 
quintillions  of  kilogrammetres  (Km).  But  for  one  unit  of 
heat  generated,  367  Km  are  consumed ;  consequently  the 
whole  rotation-effect  of  the  sun  could  only  cover  the  expendi- 
ture of  heat  for  the  space  of  183  years. 

The  space  of  our  solar  system  is  filled  with  a  great  num- 
ber of  ponderable  objects,  which  have  a  tendency  to  move 
towards  the  centre  of  gravity  of  the  sun ;  and  in  so  doing, 
their  rate  of  motion  is  more  and  more  accelerated. 

A  mass,  without  motion,  placed  within  the  sphere  of  the 
sun's  attraction,  will  obey  this  attraction,  and,  if  there  be  no 
disturbing  influences,  will  fall  in  a  straight  line  into  the  sun. 
In  reality,  however,  such  a  rectilinear  path  can  scarcely  occur, 
as  may  be  shown  by  experiment. 

Let  a  weight  be  suspended  by  a  string  so  that  it  can  only 
touch  the  floor  in  one  point.  Lift  the  weight  up  to  a  certain 
height,  and  at  the  same  time  stretch  the  string  out  to  its  full 
length ;  if  the  weight  be  now  allowed  to  fall,  it  will  be  ob- 
served, almost  in  every  case,  not  to  reach  at  once  the  point  on 
the  floor  towards  which  it  tends  to  move,  but  to  move  round 
this  point  for  some  time  in  a  curved  line. 

The  reason  of  this  phenomenon  is  that  the  slightest  devia- 
tion of  the  weight  from  its  shortest  route  towards  the  point 
on  the  floor,  caused  by  some  disturbing  influence  such  as  the 
resistance  of  the  air  against  a  not  perfectly  uniform  surface, 


268  CELESTIAL    DYNAMICS. 

will  maintain  itself  as  long  as  motion  lasts.  It  is  neverthe- 
less possible  for  the  weight  to  move  at  once  to  the  point ;  the 
probability  of  its  doing  so,  however,  becomes  the  less  as  the 
height  from  which  it  is  allowed  to  drop  increases,  or  the  string, 
by  means  of  which  it  is  suspended,  is  lengthened. 

Similar  laws  influence  the  movements  of  bodies  in  the 
space  of  the  solar  system.  The  height  of  the  fall  is  here 
represented  by  the  original  distance  from  the  sun  at  which  the 
body  begins  to  move  ;  the  length  of  the  string  by  the  sun's 
attraction,  which  increases  when  the  distance  decreases  ;  and 
the  small  surface  of  contact  on  the  floor  by  the  area  of  the 
section  of  the  sun's  sphere.  If  now  a  cosmical  mass  within 
the  physical  limits  of  the  sun's  sphere  of  attraction  begins  its 
fall  towards  that  heavenly  body,  it  will  be  disturbed  in  its 
long  path  for  many  centuries,  at  first  by  the  nearest  fixed 
stars,  and  afterwards  by  the  bodies  of  the  solar  system. 
Motion  of  such  a  mass  in  a  straight  line,  or  its  perpendicular 
fall  into  the  sun,  would,  therefore,  under  such  conditions,  be 
impossible.  The  observed  movement  of  all  planetary  bodies 
in  closed  curves  agrees  with  this. 

We  shall  now  return  to  the  example  of  the  weight  sus- 
pended by  a  string  and  oscillating  round  a  point  towards 
which  it  is  attracted.  The  diameters  of  the  orbits  described 
by  this  weight  are  observed  to  be  nearly  equal ;  continued  ob- 
servation, however,  shows  that  these  diameters  gradually  di- 
minish in  length,  so  that  the  weight  will  by  degrees  approach 
the  point  in  which  it  can  touch  the  floor.  The  weight,  how- 
ever, touches  the  floor  not  in  a  mathematical  point,  but  in  a 
small  surface  ;  as  soon,  therefore,  as  the  diameter  of  the  curve 
in  which  the  weight  moves  is  equal  to  the  diameter  of  this 
surface,  the  weight  will  touch  the  floor.  This  final  contact  is 
no  accidental  or  improbable  event,  but  a  necessary  phenome- 
non caused  by  the  resistance  which  the  oscillating  mass  con- 
stantly suffers  from  the  air  and  friction.  If  all  resistance 
could  be  annihilated,  the  motion  of  the  weight  would  of  course 
continue  in  equal  oscillations. 


MEASURE   OF  THE   SUN'S   HEAT.  269 

The  same  law  holds  good  for  celestial  bodies. 

The  movements  of  celestial  bodies  in  an  absolute  vacuum 
would  be  as  uniform  as  those  of  a  mathematical  pendulum, 
whereas  a  resisting  medium  pervading  all  space  would  cause 
the  planets  to  move  in  shorter  and  shorter  orbits,  and  at  last 
to  fall  into  the  sun. 

Assuming  such  a  resisting  medium,  these  wandering  ce- 
lestial bodies  must  have  on  the  periphery  of  the  solar  system 
their  cradle,  and  in  its  centre  their  grave  ;  and  however  long 
the  duration,  and  however  great  the  number  of  their  revolu- 
tions may  be,  as  many  masses  will  on  the  average  in  a  cer- 
tain time  arrive  at  the  sun  as  formerly  in  a  like  period  of 
time  came  within  his  sphere  of  attraction. 

All  these  bodies  plunge  with  a  violent  impetus  into  their 
common  grave.  Since  no  cause  exists  without  an  effect,  each 
of  these  cosmical  masses  will,  like  a  weight  falling  to  the 
earth,  produce  by  its  percussion  an  amount  of  heat  propor- 
tional to  its  vis  viva. 

From  the  idea  of  a  sun  whose  attraction  acts  throughout 
space,  of  ponderable  bodies  scattered  throughout  the  universe, 
and  of  a  resisting  aether,  another  idea  necessarily  follows — 
that,  namely,  of  a  continual  and  inexhaustible  generation  of 
heat  on  the  central  body  of  this  cosmical  system. 

Whether  such  a  conception  be  realized  in  our  solar  system 
— whether,  in  other  words,  the  wonderful  and  permanent  evo- 
lution of  light  and  heat  be  caused  by  the  uninterrupted  fall 
of  cosmical  matter  into  the  sun — will  now  be  more  closely 
examined. 

The  existence  of  matter  in  a  primordial  condition  (  Urma- 
terie),  moving  about  in  the  universe,  and  assumed  to  follow 
the  attraction  of  the  nearest  stellar  system,  will  scarcely  be 
denied  by  astronomers  and  physicists ;  for  the  richness  of 
surrounding  nature,  as  well  as  the  aspect  of  the  starry  heav- 
ens, prevents  the  belief  that  the  wide  space  which  separates 
our  solar  system  from  the  regions  governed  by  the  other  fixed 


270  CELESTIAL    DYNAMICS. 

stars  is  a  vacant  solitude  destitute  of  matter.  We  shall 
leave,  however,  all  suppositions  concerning  subjects  so  distant 
from  us  both  in  time  and  space,  and  confine  our  attention  ex- 
clusively to  what  may  be  learnt  from  the  observation  of  the 
existing  state  of  things. 

Besides  the  fourteen  known  planets  with  their  eighteen 
satellites,  a  great  many  other  cosmical  masses  move  within 
the  space  of  the  planetary  system,  of  which  the  comets  de- 
serve to  be  mentioned  first. 

Kepler's  celebrated  statement  that  "  there  are  more  com- 
ets in  the  heavens  than  fish  in  the  ocean,"  is  founded  on  the 
fact  that,  of  all  the  comets  belonging  to  our  solar  system, 
comparatively  few  can  be  seen  by  the  inhabitants  of  the  earth, 
and  therefore  the  not  inconsiderable  number  of  actually  ob- 
served comets  obliges  us,  according  to  the  rules  of  the  calcu- 
lus of  probabilities,  to  assume  the  existence  of  a  great  many 
more  beyond  the  sphere  of  our  vision. 

Besides  planets,  satellites,  and  comets,  another  class  of 
celestial  bodies  exists  within  our  solar  system.  These  are 
masses  which,  on  account  of  their  smallness,  may  be  consid- 
ered as  cosmical  atoms,  and  which  Arago  has  appropriately 
called  asteroids.  They,  like  the  planets  and  the  comets,  are 
governed  by  gravity,  and  move  in  elliptical  orbits  round  the 
sun.  When  accident  brings  them  into  the  immediate  neigh- 
bourhood of  the  earth,  they  produce  the  phenomena  of  shoot- 
ing-stars and  fireballs. 

It  has  been  shown  by  repeated  observation,  that  on  a 
bright  night  twenty  minutes  seldom  elapse  without  a  shooting- 
star  being  visible  to  an  observer  in  any  situation.  At  certain 
times  these  meteors  are  observed  in  astonishingly  great  num- 
bers ;  during  the  meteoric  shower  at  Boston,  which  lasted 
nine  hours,  when  they  were  said  to  fall  "  crowded  together 
like  snow-flakes,"  they  were  estimated  as  at  least  240,000. 
On  the  whole,  the  number  of  asteroids  which  come  near  the 
earth  in  the  space  of  a  year  must  be  computed  to  be  many 


MEASUEE   OF   THE   SUN'S   HEAT.  271 

thousands  of  millions.  This,  without  doubt,  is  only  a  small 
fraction  of  the  number  of  asteroids  that  move  round  the  sun, 
which  number,  according  to  the  rules  of  the  calculus  of  prob- 
abilities, approaches  the  infinite. 

As  has  been  already  stated,  on  the  existence  of  a  resisting 
sether  it  depends  whether  the  celestial  bodies,  the  planets,  the 
.comets,  and  the  asteroids  move  at  constant  mean  distances 
round  the  sun,  or  whether  they  are  constantly  approaching 
that  central  body. 

Scientific  men  do  not  doubt  the  existence  of  such  an  aether. 
Littrow,  amongst  others,  expresses  himself  on  this  point  as 
follows  : — "  The  assumption  that  the  planets  and  the  comets 
move  in  an  absolute  vacuum  can  in  no  way  be  admitted. 
Even  if  the  space  between  celestial  bodies  contained  no  other 
matter  than  that  necessary  for  the  existence  of  light  (whether 
light  be  considered  as  emission  of  matter  or  the  undulations 
of  a  universal  aether),  this  alone  is  sufficient  to  alter  the  mo- 
tion of  the  planets  in  the  course  of  time  and  the  arrangement 
of  the  whole  system  itself;  the  fall  of  all  the  planets  and  the 
comets  into  the  sun  and  the  destruction  of  the  present  state 
of  the  solar  system  must  be  the  final  results  of  this  action." 

A  direct  proof  of  the  existence  of  such  a  resisting  medium 
has  been  furnished  by  the  academician  Encke.  He  found 
that  the  comet  named  after  him,  which  revolves  round  the 
sun  in  the  short  space  of  1207  days,  shows  a  regular  acceler- 
ation of  its  motion,  in  consequence  of  which  the  time  of  each 
revolution  is  shortened  by  about  six  hours. 

From  the  great  density  and  magnitude  of  the  planets,  the 
shortening  of  the  diameters  of  their  orbits  proceeds,  as  might 
be  expected,  very  slowly,  and  is  up  to  the  present  time  inap- 
preciable. The  smaller  the  cosmical  masses  are,  on  the  con- 
trary, other  circumstances  remaining  the  same,  the  faster  they 
move  towards  the  sun ;  it  may  therefore  happen  that  in  a 
space  of  time  wherein  the  mean  distance  of  the  earth  from 
the  sun  would  diminish  one  metre,  a  small  asteroid 


272  CELESTIAL   DYNAMICS. 

travel  more  than  one  thousand  miles  towards  the  central 
body. 

As  cosmical  masses  stream  from  all  sides  in  immense 
numbers  towards  the  sun,  it  follows  that  they  must  become 
more  and  more  crowded  together  as  they  approach  thereto. 
The  conjecture  at  once  suggests  itself  that  the  zodiacal  light, 
the  nebulous  light  of  vast  dimensions  which  surrounds  the 
sun,  owes  its  origin  to  such  closely-packed  asteroids.  How- 
ever it  may  be,  this  much  is  certain,  that  this  phenomenon  is 
caused  by  matter  which  moves  according  to  the  same  laws  as 
the  planets  round  the  sun,  and  it  consequently  follows  that 
the  whole  mass  which  originates  the  zodiacal  light  is  contin- 
ually approaching  the  sun  and  falling  into  it. 

This  light  does  not  surround  the  sun  uniformly  on  all 
sides ;  that  is  to  say,  it  has  not  the  form  of  a  sphere,  but  that 
of  a  thin  convex  lens,  the  greater  diameter  of  which  is  in  the 
plane  of  the  solar  equator,  and  accordingly  it  has  to  an  ob- 
server on  our  globe  a  pyramidal  form.  Such  lenticular  dis- 
tribution of  the  masses  in  the  universe  is  repeated  in  a  re- 
markable manner  in  the  disposition  of  the  planets  and  the 
fixed  stars. 

From  the  great  number  of  cometary  masses  and  asteroids 
and  the  zodiacal  light  on  the  one  hand,  and  the  existence  of  a 
resisting  aether  on  the  other,  it  necessarily  follows  that  pon- 
derable matter  must  continually  be  arriving  on  the  solar  sur- 
face. The  effect  produced  by  these  masses  evidently  depends 
on  their  final  velocity ;  and,  in  order  to  determine  the  latter,  we 
shall  discuss  some  of  the  elements  of  the  theory  of  gravitation. 

The  final  velocity  of  a  weight  attracted  by  and  moving 
towards  a  celestial  body  will  become  greater  as  the  height 
through  which  the  weight  falls  increases.  This  velocity,  how- 
ever, if  it  be  only  produced  by  the  fall,  cannot  exceed  a  cer- 
tain magnitude ;  it  has  a  maximum,  the  value  of  which  de- 
pends on  the  volume  and  mass  of  the  attracting  celestial  body. 


273 

Let  r  be  the  radius  of  a  spherical  and  solid  celestial  body, 
and  g  the  velocity  at  the  end  of  the  first  second  of  a  weight 
falh'ng  on  the  surface  of  this  body ;  then  the  greatest  velocity 
which  this  weight  can  obtain  by  its  fall  towards  the  celestial 
body,  or  the  velocity  with  which  it  will  arrive  at  its  surface 
after  a  fall  from  an  infinite  height,  is  \/2gr  in  one  second. 
This  number,  wherein  g  and  r  are  expressed  in  metres,  we 
shall  call  G. 

For  our  globe  the  value  of  g  is  9*8164  .  .  and  that  of  r 
6,369,800  ;  and  consequently  on  our  earth 

G  =  47(2  x  9*8164X6,369,800)  =  11,183. 

The  solar  radius  is  112*05  times  that  of  the  earth,  and  the 
velocity  produced  by  gravity  on  the  sun's  surface  is  28*36 
times  greater  than  the  same  velocity  on  the  surface  of  our 
globe  ;  the  greatest  velocity  therefore  which  a  body  could  ob- 
tain in  consequence  of  the  solar  attraction,  or 

G=  |/(28*36X  112*05)  X  11,183  =  630,400; 
that  is,  this  maximum  velocity  is  equal  to  630,400  metres,  or 
85  geographical  miles  in  one  second. 

By  the  help  of  this  constant  number,  which  may  be  called 
the  characteristic  of  the  solar  system,  the  velocity  of  a  body 
in  central  motion  may  easily  be  determined  at  any  point  of  its 
orbit.  Let  a  be  the  mean  distance  of  the  planetary  body  from 
the  centre  of  gravity  of  the  sun,  or  the  greater  semidiameter 
of  its  orbit  (the  radius  of  the  sun  being  taken  as  unity)  ;  and 
let  h  be  the  distance  of  the  same  body  at  any  point  of  its  orbit 
from  the  centre  of  gravity  of  the  sun ;  then  the  velocity, 
expressed  in  metres,  of  the  planet  at  the  distance  h  is 

'2o^A 
GX 


At  the  moment  the  planet  comes  in  contact  with  the  solar  sur- 
face, h  is  equal  to  1,  and  its  velocity  is  therefore 


12* 


274  CELESTIAL   DYNAMICS. 

It  follows  from  this  formula  that  the  smaller  2a  (or  the 
major  axis  of  the  orbit  of  a  planetary  body)  becomes,  the 
less  will  be  its  velocity  when  it  reaches  the  sun.  This  velo- 
city, like  the  major  axis,  has  a  minimum ;  for  so  long  as  the 
planet  moves  outside  the  sun,  its  major  axis  cannot  be  shorter 
than  the  diameter  of  the  sun,  or,  taking  the  solar  radius  as  a 
unit,  the  quantity  2a  can  never  be  less  than  2.  The  smallest 
velocity  with  which  we  can  imagine  a  cosmical  body  to  arrive 
on  the  surface  of  the  sun  is  consequently 

^=445,750, 

or  a  velocity  of  60  geographical  miles  in  one  second. 

For  this  smallest  value  the  orbit  of  the  asteroid  is  circu- 
lar ;  for  a  larger  value  it  becomes  elliptical,  until  finally,  with 
increasing  excentricity,  when  the  value  of  2a  approaches  in- 
finity, the  orbit  becomes  a  parabola.  In  the  last  case  the 
velocity  is 


or,  85  geographical  miles  in  one  second. 

If  the  value  of  the  major  axis  become  negative,  or  the 
orbit  assume  the  form  of  a  hyperbola,  the  velocity  may  in- 
crease without  end.  But  this  could  only  happen  when  cosmi- 
cal masses  enter  the  space  of  the  solar  system  with  a  pro- 
jected velocity,  or  when  masses,  having  missed  the  sun's  sur- 
face, move  into  the  universe  and  never  return ;  hence  a  ve- 
locity greater  than  Gr  can  only  be  regarded  as  a  rare  excep- 
tion, and  we  shall  therefore  only  consider  velocities  comprised 
within  the  limits  of  60  and  80  miles.* 

The   final  velocity  with   which  a  weight   moving   in  a 

*  The  relative  velocity  also  with  which  an  asteroid  reaches  the  solar 
surface  depends  in  some  degree  on  the  velocity  of  the  sun's  rotation.  This, 
however,  as  well  as  the  rotatory  effect  of  the  asteroid,  is  without  moment, 
and  may  be  neglected. 


MEASURE   OF   THE   SUN's   HEAT.  275 

straight  line  towards  the  centre  of  the  sun  arrives  at  the  solar 
surface  is  expressed  by  the  formula 


wherein  c  expresses  the  final  velocity  in  metres,  and  h  the 
original  distance  from  the  centre  of  the  sun  in  terms  of  solar 
radius.  If  this  formula  be  compared  with  the  foregoing,  it 
will  be  seen  that  a  mass  which,  after  moving  in  central  mo- 
tion, arrives  at  the  sun's  surface  has  the  same  velocity  as  it 
would  possess  had  it  fallen  perpendicularly  into  the  sun  from 
a  distance*  equal  to  the  major  axis  of  its  orbit ;  whence  it  is 
apparent  that  a  planet,  on  arriving  at  the  sun,  moves  at  least 
as  quickly  as  a  weight  which  falls  freely  towards  the  sun 
from  a  distance  as  great  as  the  solar  radius,  or  96,000  geo- 
graphical miles. 

What  thermal  effect  corresponds  to  such  velocities?  Is 
the  effect  sufficiently  great  to  play  an  important  part  in  the 
immense  development  of  heat  on  the  sun  ? 

This  crucial  question  may  be  easily  answered  by  help  of 
the  preceding  considerations.  According  to  the  formula  given 
at  the  end  of  Chapter  II.,  the  degree  of  heat  generated  by 
percussion  is 

=  0-000139°  Xc2, 

where  c  denotes  the  velocity  of  the  striking  body  expressed  in 
metres.  The  velocity  of  an  asteroid  when  it  strikes  the  sun 
measures  from  445,750  to  630,400  metres  ;  the  caloric  effect 
of  the  percussion  is  consequently  equal  to  from  27|  to  55  mil- 
lions of  degrees  of  heatf. 

An  asteroid,  therefore,  by  its  fall  into  the  sun  developes 

*  This  distance  is  to  be  counted  from  the  centre  of  the  sun. 

f  Throughout  this  memoir  the  degrees  of  heat  are  expressed  in  the 
Centigrade  scale.  Unless  stated  to  the  contrary,  the  measures  of  length 
are  given  in  geographical  miles.  A  geographical  mile  —  7420  metres,  and 
an  English  mile  =  1608  metres.— TR. 


276  CELESTIAL   DYNAMICS. 

from  4600  to  9200  times  as  much  heat  as  would  be  generated 
by  the  combustion  of  an  equal  mass  of  coal. 


IV.—ORIGIN    OF   THE    SUN'S   HEAT. 

THE  question  why  the  planets  move  in  curved  orbits,  one 
of  the  grandest  of  problems,  was  solved  by  Newton  in  con- 
sequence, it  is  believed,  of  his  reflecting  on  the  fall  of  an  ap- 
ple. This  story  is  not  improbable,  for  we  are  on  the  right 
track  for  the  discovery  of  truth  when  once  we  clearly  recog- 
nize that  between  great  and  small  no  qualitative  but  only  a 
quantitative  difference  exists — when  we  resist  the  suggestions 
of  an  ever  active  imagination,  and  look  for  the  same  laws  in 
the  greatest  as  well  as  in  the  smallest  processes  of  nature. 

This  universal  range  is  the  essence  of  a  law  of  nature, 
and  the  touchstone  of  the  correctness  of  human  theories.  "We 
observe  the  fall  of  an  apple,  and  investigate  the  law  which 
governs  this  phenomenon ;  for  the  earth  we  substitute  the 
sun,  and  for  the  apple  a  planet,  and  thus  possess  ourselves  of 
the  key  to  the  mechanics  of  the  heavens. 

•  As  the  same  laws  prevail  in  the  greater  as  well  as  in  the 
smaller  processes  of  nature,  Newton's  method  may  be  used  in 
solving  the  problem  of  the  origin  of  the  sun's  heat.  We 
know  the  connexion  between  the  space  through  which  a  body 
falls,  the  velocity,  the  vis  viva,  and  the  generation  of  heat  on 
the  surface  of  this  globe ;  if  we  again  substitute  for  the  earth 
the  sun,  with  a  mass  350,000  greater,  and  for  a  height  of  a 
few  metres  celestial  distances,  we  obtain  a  generation  of  heat 
exceeding  all  terrestrial  measures.  And  since  we  have  suffi- 
cient reason  to  assume  the  actual  existence  of  such  mechani- 
cal processes  in  the  heavens,  we  find  therein  the  only  tenable 
explanation  of  the  origin  of  the  heat  of  the  sun. 


ORIGIN   OF   TNE   SUN'S   HEAT.  277 

The  fact  that  the  development  of  heat  by  mechanical 
means  on  the  surface  of  our  globe  is,  as  a  rule,  not  so  great, 
and  cannot  be  so  great  as  the  generation  of  the  same  agent 
by  chemical  means,  as  by  combustion,  follows  from  the  laws 
already  discussed ;  and  this  fact  cannot  be  used  as  an  argu- 
ment against  the  assumption  of  a  greater  development  of 
heat  by  a  greater  expenditure  of  mechanical  work.  It  has 
been  shown  that  the  heat  generated  by  a  weight  falling  from 
a  height  of  367  metres  is  only  ^th  part  of  the  heat  pro- 
duced by  the  combustion  of  the  same  weight  of  coal ;  just  as 
small  is  the  amount  of  heat  developed  by  a  weight  moving 
with  the  not  inconsiderable  velocity  of  85  metres  in  one  sec- 
ond. But,  according  to  the  laws  of  mechanics,  the  effect  is 
proportional  to  the  square  of  the  velocity ;  if  therefore  the 
weight  move  100  times  faster,  or  with  a  velocity  of  8500 
metres  in  one  second,  it  will  produce  a  greater  effect  than  the 
combustion  of  an  equal  quantity  of  coal. 

It  is  true  that  so  great  a  velocity  cannot  be  obtained  by 
human  means  ;  everyday  experience,  however,  shows  the  de- 
velopment of  high  degrees  of  temperature  by  mechanical 
processes. 

In  the  common  flint  and  steel,  the  particles  of  steel  which 
are  struck  off  are  sufficiently  heated  to  burn  in  air.  A  few 
blows  directed  by  a  skilful  blacksmith  with  a  sledge-hammer 
against  a  piece  of  cold  metal  may  raise  the  temperature  of 
the  metal  at  the  points  of  collision  to  redness. 

The  new  crank  of  a  steamer,  whilst  being  polished  by 
friction,  becomes  red-hot,  several  buckets  of  water  being  re- 
quired to  cool  it  down  to  its  ordinary  temperature. 

When  a  railway  train  passes  with  even  less  than  its  ordi- 
nary velocity  along  a  very  sharp  curve  of  the  line,  sparks  are 
observed  in  consequence  of  the  friction  against  the  rails. 

One  of  the  grandest  constructions  for  the  production  of 
motion  by  human  art  is  the  channel  in  which  the  wood  was 
allowed  to  glide  down  from  the  steep  and  lofty  sides  of  Mount 


278  CELESTIAL   DYNAMICS. 

Pilatus,  into  the  plain  below.  This  wooden  channel  which 
was  built  about  thirty  years  ago  by  the  engineer  Rupp,  was 
9  English  miles  in  length ;  the  largest  trees  were  shot  down 
it  from  the  top  to  the  bottom  of  the  mountain  in  about  two 
minutes  and  a  half.  The  momentum  possessed  by  the  trees 
on  their  escaping  at  their  journey's  end  from  the  channel  was 
sufficiently  great  to  bury  their  thicker  ends  in  the  ground  to 
the  depth  of  from  6  to  8  metres.  To  prevent  the  wood  get- 
ting too  hot  and  taking  fire,  water  was  conducted  in  many 
places  into  the  channel.  « 

This  stupendous  mechanical  process,  when  compared  with 
cosmical  processes  on  the  sun,  appears  infinitely  small.  In 
the  latter  case  it  is  the  mass  of  the  sun  which  attracts,  and 
in  lieu  of  the  height  of  Mount  Pilatus  we  have  distances  of  a 
hundred  thousand  and  more  miles  ;  the  amount  of  heat  gene- 
rated by  cosmical  falls  is  therefore  at  least  9  million  times 
greater  than  in  our  terrestrial  example. 

Rays  of  heat  on  passing  through  glass  and  other  transpa- 
rent bodies  undergo  partial  absorption,  which  differs  in  de- 
gree, however,  according  to  the  temperature  of  the  source 
from  which  the  heat  is  derived.  Heat  radiated  from  sources 
less  warm  than  boiling  water  is  almost  completely  stopped  by 
thin  plates  of  glass.  As  the  temperature  of  a  source  of  heat 
increases,  its  rays  pass  more  copiously  through  diathermic 
bodies.  A  plate  of  glass,  for  example,  weakens  the  rays  of 
a  red-hot  substance,  even  when  the  latter  is  placed  very  close 
to  it,  much  more  than  it  does  those  emanating  at  a  much 
greater  distance  from  a  white-hot  body.  If  the  quality  of 
the  sun's  rays  be  examined  in  this  respect,  their  diathermic 
energy  is  found  to  be  far  superior  to  that  of  all  artificial 
sources  of  heat.  The  temperature  of  the  focus  of  a  concave 
metallic  reflector  in  which  the  sun's  light  has  been  collected 
is  only  diminished  from  one-seventh  to  one-eighth  by  the  in- 
terposition of  a  screen  of  glass.  If  the  same  experiment  be 


ORIGIN   OF  THE   SUN's   HEAT.  279 

made  with  an  artificial  and  luminous  source  of  heat,  it  is 
found  that,  though  the  focus  be  very  hot  when  the  screen  is 
away,  the  interposition  of  the  latter  cuts  off  nearly  all  the 
heat ;  moreover,  the  focus  will  not  recover  its  former  temper- 
ature when  reflector  and  screen  are  placed  sufficiently  near  to 
the  source  of  heat  to  make  the  focus  appear  brighter  than  it 
did  in  the  former  position  without  the  glass  screen. 

The  empirical  law,  that  the  diathermic  energy  of  heat  in- 
creases with  the  temperature  of  the  source  from  which  the 
heat  is  radiated,  teaches  us  that  the  sun's  surface  must  be 
much  hotter  than  the  most  powerful  process  of  combustion 
could  render  it. 

Other  methods  furnish  the  same  conclusion.  If  we  ima- 
gine the  sun  to  be  surrounded  by  a  hollow  sphere,  it  is  clear 
that  the  inner  surface  of  this  sphere  must  receive  all  the  heat 
radiated  from  the  sun.  At  the  distance  of  our  globe  from  the 
sun,  such  a  sphere  would  have  a  radius  215  times  as  great, 
and  an  area  46,000  times  as  large  as  the  sun  himself;  those 
luminous  and  calorific  rays,  therefore,  which  meet  this  spheri- 
cal surface  at  right  angles  retain  only  j^ih.  part  of  their 
original  intensity.  If  it  be  further  considered  that  our  at- 
mosphere absorbs  a  part  of  the  solar  rays,  it  is  clear  that  the 
rays  which  reach  the  tropics  of  our  earth  at  noonday  can 
only  possess  from  j^oth  to  ^oooth  °f  tne  power  with  which 
they  started.  These  rays,  when  gathered  from  a  surface  of 
from  5  to  6  square  metres,  and  concentrated  in  an  area  of 
one  square  centimetre,  would  produce  about  the  temperature 
which  exists  on  the  sun,  a  temperature  more  than  sufficient 
to  vaporize  platinum,  rhodium,  and  similar  metals. 

The  radiation  calculated  in  Chapter  III.  likewise  proves 
the  enormous  temperature  of  the  solar  surface.  From  the 
determination  mentioned  therein,  it  follows  that  each  square 
centimetre  of  the  sun's  surface  loses  by  radiation  about  80 
units  of  heat  per  minute — an  immense  quantity  in  compari- 
son with  terrestrial  radiations. 


280  CELESTIAL   DYNAMICS. 

A  correct  theory  of  the  origin  of  the  sun's  heat  must  ex- 
plain the  cause  of  such  enormous  temperatures.  This  expla- 
nation can  be  deduced  from  the  foregoing  statements.  Ac- 
cording to  Pouillet,  the  temperature  at  which  bodies  appear 
intensely  white-hot  is  about  1500°  C.  The  heat  generated  by 
the  combustion  of  one  kilogramme  of  hydrogen  is,  as  deter- 
mined by  Dulong,  34,500,  and  according  to  the  more  recent 
experiments  of  Grassi,  34,666  units  of  heat.  One  part  of 
hydrogen  combines  with  eight  parts  of  oxygen  to  form  water  ; 
hence  one  kilogramme  of  these  two  gases  mixed  in  this  ratio 
would  produce  3850°. 

Let  us  now  compare  this  heat  with  the  amount  of  the 
same  agent  generated  by  the  fall  of  an  asteroid  into  the  sun. 
Without  taking  into  account  the  low  specific  heat  of  such 
masses  when  compared  with  that  of  water,  we  find  the  heat 
developed  by  the  asteroid  to  be  from  7000  to  15,000  times 
greater  than  that  of  the  oxyhydrogen  mixture.  From  data 
like  these,  the  extraordinary  diathermic  energy  of  the  sun's 
rays,  the  immense  radiation  from  his  surface,  and  the  high 
temperature  in  the  focus  of  the  reflector  are  easily  accounted 
for. 

The  facts  above  mentioned  show  that,  unless  we  assume 
on  the  sun  the  existence  of  matter  with  unheard  of  chemical 
properties  as  a  deus  ex  machind,  no  chemical  process  could 
maintain  the  present  high  radiation  of  the  sun ;  it  also  fol- 
lows from  the  above  results,  that  the  chemical  nature  of  bo- 
dies which  fall  into  the  sun  does  not  in  the  least  affect  our 
conclusions ;  the  effect  produced  by  the  most  inflammable 
substance  would  not  differ  by  one-thousandth  part  from  that 
resulting  from  the  fall  of  matter  possessing  but  feeble  chemi- 
cal affinities.  As  the  brightest  artificial  light  appears  dark  in 
comparison  with  the  sun's  light,  so  the  mechanical  processes 
of  the  heavens  throw  into  the  shade  the  most  powerful  chem- 
ical actions. 

The  quality  of  the  sun's  rays,  as  dependent  on  his  temper- 


ORIGIN   OF   THE   SUN's   HEAT.  281 

ature,  is  of  the  greatest  importance  to  mankind.  If  the  solar 
heat  were  originated  by  a  chemical  process,  and  amounted 
near  its  source  to  a  temperature  of  a  few  thousand  degrees, 
it  would  be  possible  for  the  light  to  reach  us,  whilst  the 
greater  part  of  the  more  important  calorific  rays  would  be  ab- 
sorbed by  the  higher  strata  of  our  atmosphere  and  then  re- 
turned to  the  universe. 

In  consequence  of  the  high  temperature  of  the  sun,  how- 
ever, our  atmosphere  is  highly  diathermic  to  his  rays,  so  that 
the  latter  reach  the  surface  of  our  earth  and  warm  it.  The 
comparatively  low  temperature  of  the  terrestrial  surface  is 
the  cause  why  the  heat  cannot  easily  radiate  back  through  the 
atmosphere  into  the  universe.  The  atmosphere  acts,  there- 
fore, like  an  envelope,  which  is  easily  pierced  by  the  solar 
rays,  but  which  offers  considerable  resistance  to  the  radiant 
heat  escaping  from  our  earth  ;  its  action  resembles  that  of  a 
valve  which  allows  liquid  to  pass  freely  in  one,  but  stops  the 
flow  in  the  opposite  direction. 

The  action  of  the  atmosphere  is  of  the  greatest  impor- 
tance as  regards  climate  and  meteorological  processes.  It 
must  raise  the  mean  temperature  of  the  earth's  surface.  Af- 
ter the  setting  of  the  sun — in  fact,  in  all  places  where  his 
rays  do  not  reach  the  surface,  the  temperature  of  the  earth 
would  soon  be  as  low  as  that  of  the  universe,  if  the  atmos- 
phere were  removed,  or  if  it  did  not  exist.  Even  the  power- 
ful solar  rays  in  the  tropics  would  be  unable  to  preserve  wa- 
ter in  its  liquid  state. 

Between  the  great  cold  which  would  reign  at  all  times 
and  in  all  places,  and  the  moderate  warmth  which  in  reality 
exists  on  our  globe,  intermediate  temperatures  may  be  ima- 
gined ;  and  it  is  easily  seen  that  the  mean  temperature  would 
decrease  if  the  atmosphere  were  to  become  more  and  more 
rare.  Such  a  rarefaction  of  a  valve-like  acting  atmosphere 
actually  takes  place  as  we  ascend  higher  and  higher  above 


282  CELESTIAL   DYNAMICS. 

the  level  of  the  sea,  and  it  is  accordingly  and  necessarily  ac- 
companied by  a  corresponding  diminution  of  temperature. 

This  well-known  fact  of  the  lower  mean  temperature  of 
places  of  greater  altitude  has  led  to  the  strangest  hypotheses. 
The  sun's  rays  were  not  supposed  to  contain  all  the  conditions 
for  warming  a  body,  but  to  set  in  motion  the  "substance" 
of  heat  contained  in  the  earth.  This  "substance"  of  heat, 
cold  when  at  rest,  was  attracted  by  the  earth,  and  was  there- 
fore found  in  greater  abundance  near  the  centre  of  the  globe. 
This  view,  it  was  thought,  explained  why  the  warming  power 
of  the  sun  was  so  much  weaker  at  the  top  of  a  mountain  than 
at  the  bottom,  and  why,  in  spite  of  his  immense  radiation,  he 
retained  his  full  powers. 

This  belief,  which  especially  prevails  amongst  imperfectly 
informed  people,  and  which  will  scarcely  succumb  to  correct 
views,  is  directly  contradicted  by  the  excellent  experiments 
made  by  Pouillet  at  different  altitudes  with  the  pyrheliometer. 
These  experiments  show  that,  everything  else  being  equal, 
the  generation  of  heat  by  the  solar  rays  is  more  powerful  in 
higher  altitudes  than  near  the  surface  of  our  globe,  and  that 
consequently  a  portion  of  these  rays  is  absorbed  on  their  pas- 
sage through  the  atmosphere.  Why,  in  spite  of  this  partial 
absorption,  the  mean  temperature  of  low  altitudes  is  never- 
theless higher  than  it  is  in  more  elevated  positions,  is  ex- 
plained by  the  fact  that  the  atmosphere  stops  to  a  far  greater 
degree  the  calorific  rays  emanating  from  the  earth  than  it 
does  those  from  the  sun. 


V.— CONSTANCY   OF   THE    SUN'S    MASS. 

NEWTON,  as  is  well  known,  considered  light  to  be  the 
emission  of  luminous  particles  from  the  sun.  In  the  contin- 
ued emission  of  light  this  great  philosopher  saw  a  cause  tend- 


CONSTANCY   OF   THE   SUN'S  MASS.  283 

ing  to  diminish  the  solar  mass ;  and  he  assumed,  in  order  to 
make  good  this  loss,  comets  and  other  cosmical  masses  to  be 
continually  falling  into  the  central  body. 

If  we  express  this  view  of  Newton's  in  the  language  of 
the  undulatory  theory,  which  is  now  universally  accepted,  we 
obtain  the  results  developed  in  the  preceding  pages.  It  is 
true  that  our  theory  does  not  accept  a  peculiar  "  substance" 
of  light  or  of  heat ;  nevertheless,  according  to  it,  the  radia- 
tion of  light  and  heat  consists  also  in  purely  material  pro- 
cesses, in  a  sort  of  motion,  in  the  vibrations  of  ponderable 
resisting  substances.  Quiescence  is  darkness  and  death  ;  mo- 
tion is  light  and  life. 

An  undulating  motion  proceeding  from  a  point  or  a  plane 
and  excited  in  an  unlimited  medium,  cannot  be  imagined  apart 
from  another  simultaneous  motion,  a  translation  of  the  parti- 
cles themselves  ;*  it  therefore  follows,  not  only  from  the  emis- 
sion, but  also  from  the  undulatory  theory,  that  radiation  con- 
tinually diminishes  the  mass  of  the  sun.  Why,  nevertheless, 
the  mass  of  the  sun  does  not  really  diminish  has  already  been 
stated. 

The  radiation  of  the  sun  is  a  centrifugal  action  equivalent 
to  a  centripetal  motion. 

The  caloric  effect  of  the  centrifugal  action  of  the  sun  can 
be  found  by  direct  observation ;  it  amounts,  according  to 
Chapter  III.,  in  one  minute  to  12,650  millions  of  cubic  miles 
of  heat,  or  5*17  quadrillions  of  units  of  heat.  In  Chapter 
IV.  it  has  been  shown  that  one  kilogramme  of  the  mass  of  an 
asteroid  originates  from  27*5  to  55  millions  of  units  of  heat ; 
the  quantity  of  cosmical  masses,  therefore,  which  falls  every 
minute  into  the  sun  amounts  to  from  94,000  to  188,000  bil- 
lions of  kilogrammes. 

To  obtain  this  remarkable  result,  we  made  use  of  a  method 

*  This  centrifugal  motion  is  perhaps  the  cause  of  the  repulsion  of  the 
tails  on  comets  when  in  the  neighbourhood  of  the  sun,  as  observed  by 


284  CELESTIAL  DYNAMICS. 

which  is  common  in  physical  inquiries.  Observation  of  the 
moon's  motion  reveals  to  us  the  external  form  of  the  earth. 
The  physicist  determines  with  the  torsion-balance  the  weight 
of  a  planet,  just  as  the  merchant  finds  the  weight  of  a  parcel 
of  goods,  whilst  the  pendulum  has  become  a  magic  power  in 
the  hands  of  the  geologist,  enabling  him  to  discover  cavities 
in  the  bowels  of  the  earth.  Our  case  is  similar  to  these.  By 
observation  and  calculation  of  the  velocity  of  sound  in  our 
atmosphere,  we  obtain  the  ratio  of  the  specific  heat  of  air  un- 
der constant  pressure  and  under  constant  volume,  and  by  the 
help  of  this  number  we  determine  the  quantity  of  heat  gene- 
rated by  mechanical  work.  The  heat  wEich  arrives  from  the 
sun  in  a  given  time  on  a  small  surface  of  our  globe  serves  as 
a  basis  for  the  calculation  of  the  whole  radiating  effect  of  the 
sun ;  and  the  result  of  a  series  of  observations  and  well- 
founded  conclusions  is  the  quantitative  determination  of  those 
cosmical  masses  which  the  sun  receives  from  the  space 
through  which  he  sends  forth  his  rays. 

Measured  by  terrestrial  standards,  the  ascertained  number 
of  so  many  billions  of  kilogrammes  per  minute  appears  in- 
credible. This  quantity,  however,  may  be  brought  nearer  to 
our  comprehension  by  comparison  with  other  cosmical  mag- 
nitudes. The  nearest  celestial  body  to  us  (the  moon)  has  a 
mass  of  about  90,000  trillions  of  kilogrammes,  and  it  would 
therefore  cover  the  expenditure  of  the  sun  for  from  one  to 
two  years.  The  mass  of  the  earth  would  afford  nourishment 
to  the  sun  for  a  period  of  from  60  to  120  years. 

To  facilitate  the  appreciation  of  the  masses  and  the  dis- 
tances occurring  in  the  planetary  system,  Herschel  draws  the 
following  picture.  Let  the  sun  be  represented  by  a  globe  1 
metre  in  diameter.  The  nearest  planet  (Mercury)  will  be 
about  as  large  as  a  pepper-corn,  3£  millimetres  in  thickness, 
at  a  distance  of  40  metres.  78  and  107  metres  distant  from 
the  sun  will  move  Venus  and  the  Earth,  each  9  millimetres  in 
diameter,  or  a  little  larger  than  a  pea.  Not  much  more  than 


285 

a  quarter  of  a  metre  from  the  Earth  will  be  the  Moon,  the 
size  of  a  mustard  seed,  2|  millimetres  in  diameter.  Mars, 
at  a  distance  of  160  metres,  will  have  about  half  the  diame- 
ter of  the  Earth  ;  and  the  smaller  planets  (Vesta,  Hebe,  As- 
trea,  Juno,  Pallas,  Ceres,  &c.),  at  a  distance  of  from  250  to 
300  metres  from  the  sun,  will  resemble  particles  of  sand. 
Jupiter  and  Saturn,  560  and  1000  metres  distant  from  the  cen- 
tre, will  be  represented  by  oranges,  10  and  9  centimetres  in 
diameter.  Uranus,  of  the  size  of  a  nut  4  centimetres  across, 
will  be  2000  metres ;  and  Neptune,  as  large  as  an  apple  6 
centimetres  in  diameter,  will  be  nearly  twice  as  distant,  or 
about  half  a  geographical  mile  away  from  the  sun.  From 
Neptune  to  the  nearest  fixed  star  will  be  more  than  2000  geo- 
graphical miles. 

To  complete  this  picture,  it  is  necessary  to  imagine  finely- 
divided  matter  grouped  in  a  diversified  manner,  moving  slowly 
and  gradually  towards  the  large  central  globe,  and  on  its  ar- 
rival attaching  itself  thereto ;  this  matter,  when  favourably 
illuminated  by  the  sun,  represents  itself  to  us  as  the  zodiacal 
light.  This  nebulous  substance  forms  also  an  important  part 
of  a  creation  in  which  nothing  is  by  chance,  but  wherein  all 
is  arranged  with  Divine  foresight  and  wisdom. 

The  surface  of  the  sun  measures  115,000  millions  of 
square  miles,  or  6£  trillions  of  square  metres ;  the  mass  of 
matter  which  in  the  shape  of  asteroids  falls  into  the  sun  every 
minute  is  from  94,000  to  188,000  billions  of  kilogrammes ; 
one  square  metre  of  solar  surface,  therefore,  receives  on  an 
average  from  15  to  30  grammes  of  matter  per  minute. 

To  compare  this  process  with  a  terrestrial  phenomenon,  a 
gentle  rain  may  be  considered  which  sends  down  in  one  hour 
a  layer  of  water  1  millimetre  in  thickness  (during  a  thunder- 
storm the  rainfall  is  often  from  ten  to  fifteen  times  this  quan- 
tity) ;  this  amounts  on  a  square  metre  to  17  grammes  per 
minute. 


286  CELESTIAL   DYNAMICS. 

The  continual  bombardment  of  the  sun  by  these  cosmical 
masses  ought  to  increase  its  volume  as  well  as  its  mass,  if 
centripetal  action  only  existed.  The  increase  of  volume, 
could  scarcely  be  appreciated  by  man  ;  for  if  the  specific  grav- 
ity of  these  cosmical  masses  be  assumed  to  be  the  same  as 
that  of  the  sun,  the  enlargement  of  his  apparent  diameter  to 
the  extent  of  one  second,  the  smallest  appreciable  magnitude, 
would  require  from  33,000  to  66,000  years. 

Not  quite  so  inappreciable  would  be  the  increase  of  the 
mass  of  the  sun.  If  this  mass,  or  the  weight  of  the  sun, 
were  augmented,  an  acceleration  of  the  motion  of  the  planets 
in  their  orbits  would  be  the  consequence,  whereby  their  times 
of  revolution  round  the  central  body  would  be  shortened. 
The  mass  of  the  sun  is  2*1  quintillions  of  kilogrammes  ;  and 
the  mass  of  the  cosmical  matter  annually  arriving  at  the  sun 
stands  to  the  above  as  1  to  from  21  —  42  millions.  Such  an 
augmentation  of  the  weight  of  the  sun  ought  to  shorten  the 
sidereal  year  from  4^000,000^1  to  8-H^booth  of  its  length,  or  from 
fths  to  f  ths  of  a  second. 

The  observations  of  astronomers  do  not  agree  with  this 
conclusion ;  we  must  therefore  fall  back  on  the  theory  men- 
tioned at  the  beginning  of  this  chapter,  which  assumes  that 
the  sun,  like  the  ocean,  is  constantly  losing  and  receiving 
equal  quantities  of  matter.  This  harmonizes  with  the  suppo- 
sition that  the  vis  viva  of  the  universe  is  a  constant  quantity. 


VI.— THE    SPOTS    ON   THE    SUN'S   DISC. 

THE  solar  disc  presents,  according  to  Sir  John  Herschel, 
the  following  appearance.  "  When  the  sun  is  observed 
through  a  powerful  telescope  provided  with  coloured  glasses 
in  order  to  lessen  the  heat  and  brightness  which  would  be 


THE   SPOTS   ON   THE   SUN's   DISC.  287 

hurtful  to  the  eyes,  large  dark  spots  are  often  seen  surrounded 
by  edges  which  are  not  quite  so  dark  as  the  spots  themselves, 
and  which  are  called  penumbras.  These  spots,  however,  are 
neither  permanent  nor  unchangeable.  When  observed  from 
day  to  day,  or  even  from  hour  to  hour,  their  form  is  seen  to 
change ;  they  expand  or  contract,  and  finally  disappear ;  on 
other  parts  of  the  solar  surface  new  spots  spring  into  exist- 
ence where  none  could  be  discovered  before.  When  they  dis- 
appear, the  darker  part  in  the  middle  of  the  spot  contracts  to 
a  point  and  vanishes  sooner  than  the  edge.  Sometimes  they 
break  up  into  two  or  more  parts  that  show  all  the  signs  of 
mobility  characteristic  of  a  liquid,  and  the  extraordinary 
commotion  which  it  seems  only  possible  for  gaseous  matter  to 
possess.  The  magnitude  of  their  motion  is  very  great.  An 
arc  of  1  second,  as  seen  from  our  globe,  corresponds  to  465 
English  miles  on  the  sun's  disc ;  a  circle  of  this  diameter, 
which  measures  nearly  220,000  English  square  miles,  is  the 
smallest  area  that  can  be  seen  on  the  solar  surface.  Spots, 
however,  more  than  45,000  English  miles  in  diameter,  and, 
if  we  may  trust  some  statements,  of  even  greater  dimensions, 
have  been  observed.  For  such  a  spot  to  disappear  in  the 
course  of  six  weeks  (and  they  rarely  last  longer),  the  edges, 
whilst  approaching  each  other,  must  move  through  a  space  of 
more  than  1000  miles  per  diem. 

"  That  portion  of  the  solar  disc  which  is  free  from  spots 
is  by  no  means  uniformly  bright.  Over  it  are  scattered  small 
dark  spots  or  pores,  which  are  found  by  careful  observation 
to  be  in  a  state  of  continual  change.  The  slow  sinking  of 
some  chemical  precipitates  in  a  transparent  liquid,  when 
viewed  from  the  upper  surface  and  in  a  direction  perpendicu- 
lar thereto,  resembles  more  accurately  than  any  other  phe- 
nomenon the  changes  which  the  pores  undergo.  The  similar- 
ity is  so  striking,  in  fact,  that  one  can  scarcely  resist  the  idea 
that  the  appearances  above  described  are  owing  to  a  luminous 
medium  moving  about  in  a  non-luminous  atmosphere,  either 


288  CELESTIAL   DYNAMICS. 

like  the  clouds  in  our  air,  or  in  wide-spread  planes  and  flame- 
like  columns,  or  in  rays  like  the  aurora  borealis. 

"  Near  large  spots,  or  extensive  groups  of  them,  large 
spaces  are  observed  to  be  covered  with  peculiarly  marked 
lines  much  brighter  than  the  other  parts  of  the  surface  ;  these 
lines  are  curved,  or  deviate  in  branches,  and  are  called  faculae. 
Spots  are  often  seen  between  these  lines,  or  to  originate  there. 
These  are  in  all  probability  the  crests  of  immense  waves  in 
the  luminous  regions  of  the  solar  atmosphere,  and  bear  wit- 
ness to  violent  action  in  their  immediate  neighbourhood." 

The  changes  on  the  solar  surface  evidently  point  to  the 
action  of  some  external  disturbing  force ;  for  every  moving 
power  resident  in  the  sun  itself  ought  to  exhaust  itself  by  its 
own  action.  These  changes,  therefore,  are  no  unimportant 
confirmation  of  the  theory  explained  in  these  pages. 

At  the  same  time  it  must  be  observed  that  our  knowledge 
of  physical  heliography  is,  from  the  nature  of  the  subject, 
very  limited ;  even  the  meteorological  processes  and  other  phe- 
nomena of  our  own  planet  are  still  in  many  respects  enigmat- 
ical. For  this  reason  no  special  information  could  be  given 
about  the  manner  in  which  the  solar  surface  is  affected  by 
cosmical  masses.  However,  I  may  be  allowed  to  mention 
some  probable  conjectures  which  offer  themselves. 

The  extraordinarily  high  temperature  which  exists  on  the 
sun  almost  precludes  the  possibility  of  its  surface  being  solid ; 
it  doubtless  consists  of  an  uninterrupted  ocean  of  fiery  fluid 
matter.  This  gaseous  envelope  becomes  more  rarefied  in 
those  parts  most  distant  from  the  sun's  centre. 

As  most  substances  are  able  to  assume  the  gaseous  state 
of  aggregation  at  high  temperatures,  the-  height  of  the  sun's 
atmosphere  cannot  be  inconsiderable.  There  are,  however, 
sound  reasons  for  believing  that  the  relative  height  of  the  so- 
lar atmosphere  is  not  very  great. 

Since  the  gravity  is  28  times  greater  on  the  sun's  surface 
than  it  is  on  our  earth,  a  column  of  air  on  the  former  must 


289 


cause  a  pressure  28  times  greater  than  it  would  on  our 
globe.  This  great  pressure  compresses  air  as  much  as  a  tem- 
perature of  8000°  would  expand  it. 

In  a  still  greater  degree  than  this  increased  gravity  do  the 
qualities  peculiar  to  gases  affect  the  height  of  the  solar  atmo- 
sphere. In  consequence  of  these  properties,  the  density  of 
our  atmosphere  rapidly  diminishes  as  we  ascend,  and  increases 
as  we  descend.  Generally  speaking,  rarefaction  increases  in 
a  geometrical  progression  when  the  heights  are  in  an  arith- 
metical progression.  If  we  ascend  or  descend  2J,  5,  or  30 
miles,  we  find  our  atmosphere  10,  100,  or  a  billion  times  more 
rarefied  or  more  dense. 

This  law,  although  modified  by  the  unequal  temperatures 
of  the  different  layers  of  the  photosphere,  and  the  unknown 
chemical  nature  of  the  substances  of  which  it  is  composed, 
must  also  hold  good  in  some  measure  for  the  sun.  As,  how- 
ever, the  mean  temperature  of  the  solar  atmosphere  must 
considerably  exceed  that  of  our  atmosphere,  the  density  of  the 
former  will  not  vary  so  rapidly  with  the  height  as  the  latter 
does.  If  we  assume  this  increase  and  decrease  on  the  sun  to 
be  ten  times  slower  than  it  is  on  our  earth,  it  follows  that  at 
the  heights  of  25,  50,  and  300  miles,  a  rarefaction  of  10, 
100,  and  a  billion  times  respectively  would  be  observed.  The 
solar  atmosphere,  therefore,  does  not  attain  a  height  of  400 
geographical  miles,  or  it  cannot  be  as  much  as  ^th  of  the 
sun's  radius.  For  if  we  take  the  density  of  the  lowest  strata 
of  the  sun's  atmosphere  to  be  1000  times  greater  that  that  of 
our  own  near^the  level  of  the  sea,  a  density  greater  than  that 
of  water,  and  necessarily  too  high,  then  at  a  height  of  400 
miles  this  atmosphere  would  be  10  billion  times  less  dense 
than  the  earth's  atmosphere ;  that  is  to  say,  to  human  com- 
prehension it  has  ceased  to  exist. 

This  discussion  shows  that  the  solar  atmosphere,  in  com- 
parison with  the  body  of  the  sun,  has  only  an  insignificant 
height ;  at  the  same  time  it  may  be  remarked  that  on  the 
13 


290  CELESTIAL  DYNAMICS. 

sun's  surface,  in  spite  of  the  great  heat,  such  substances  as 
water  may  possibly  exist  in  the  liquid  state  under  a  pressure 
thousands  of  times  greater  than  that  of  our  atmosphere. 

Since  gases,  when  free  from  any  solid  particles,  emit,  even 
at  very  high  temperatures,  a  pale  transparent  light — the  so- 
called  lumen  philosophicum — it  is  probable  that  the  intense 
white  light  of  the  sun  has  its  origin  in  the  denser  parts  of  his 
surface.  If  such  be  assumed  to  be  the  case,  the  sun's  spots 
and  faculae  seem  to  be  the  disturbances  of  the  fiery  liquid 
ocean,  caused  by  most  powerful  meteoric  processes,  for  which 
all  necessary  materials  are  present,  and  partly  to  be  caused 
by  the  direct  influence  of  streams  of  asteroids.  The  deeper 
and  less  heated  parts  of  this  fiery  ocean  become  thus  exposed, 
and  perhaps  appear  to  us  as  spots,  whereas  the  elevations 
form  the  so-called  faculae. 

According  to  the  experiments  made  by  Henry,  an  Ameri- 
can physicist,  the  rays  sent  forth  from  the  spots  do  not  pro- 
duce the  same  heating  effect  as  those  emitted  by  the  brighter 
parts. 

We  have  to  mention  one  more  remarkable  circumstance. 
The  spots  appear  to  be  confined  to  a  zone  which  extends  30° 
on  each  side  of  the  sun's  equator.  The  thought  naturally 
suggests  itself  that  some  connexion  exists  between  those  solar 
processes  which  produce  the  spots  and  faculse,  the  velocity  of 
rotation  of  the  sun,  and  the  swarms  of  asteroids,  and  to  de- 
duce therefrom  the  limitation  of  the  spots  to  the  zone  men- 
tioned. It  still  remains  enigmatical  by  what  means  nature 
contrives  to  bring  about  the  uniform  radiation  which  pertains 
alike  to  the  polar  and  equatorial  regions  of  the  sun. 


THE  TIDAL   WAVE.  291 


VII — THE    TIDAL    WAVE. 

IN  almost  every  case  the  forces  and  motions  on  the  sur- 
face of  the  earth  may  be  traced  back  to  the  rays  of  the  sun. 
Some  processes,  however,  form  a  remarkable  exception. 

One  of  these  is  the  tides.  Beautiful,  and  in  some  re- 
spects exhaustive  researches  on  this  phenomenon  have  been 
made  by  Newton,  Laplace,  and  others.  The  tides  are  caused 
by  the  attraction  exercised  by  the  sun  and  the  moon  on  the 
moveable  parts  of  the  earth's  surface,  and  by  the  axial  rota- 
tion of  our  globe. 

The  alternate  rising  and  falling  of  the  level  of  the  sea 
may  be  compared  to  the  ascent  and  descent  of  a  pendulum  ' 
oscillating  under  the  influence  of  the  earth's  attraction. 

The  continual  resistance,  however  weak  it  may  be,  which 
an  instrument  of  this  nature  (a  physical  pendulum)  suffers, 
constantly  shortens  the  amplitude  of  the  oscillations  which  it 
performs ;  and  if  the  pendulum  be  required  to  continue  in 
uniform  motion,  it  must  receive  a  constant  supply  of  vis  viva 
corresponding  to  the  resistance  it  has  to  overcome. 

Clocks  regulated  by  a  pendulum  obtain  such  a  supply, 
either  from  a  raised  weight  or  a  bent  spring.  The  power 
consumed  in  raising  the  weight  or  in  bending  the  spring, 
which  power  is  represented  by  the  raised  weight  or  the  bent 
spring,  overcomes  for  a  time  the  resistance,  and  thus  secures 
the  uniform  motion  of  the  pendulum  and  clock.  In  doing  so, 
the  weight  sinks  down  or  the  spring  uncoils,  and  therefore 
force  must  be  expended  in  winding  the  clock  up  again,  or  it 
would  stop  moving. 

Essentially  the  same  holds  good  for  the  tidal  wave.  The 
moving  waters  rub  against  each  other,  against  the  shore,  and 
against  the  atmosphere,  and  thus,  meeting  constantly  with  re- 
sistance, would  soon  come  to  rest  if  a  vis  viva  did  not  exist 
competent  to  overcome  these  obstacles.  This  vis  viva  is  the 


292  CELESTIAL  DYNAMICS. 

rotation  of  the  earth  on  its  axis,  and  the  diminution  and  final 
exhaustion  thereof  will  be  a  consequence  of  such  an  action. 

The  tidal  wave  causes  a  diminution  of  the  velocity  of  the 
rotation  of  the  earth. 

This  important  conclusion  can  be  proved  in  different  ways. 

The  attraction  of  the  sun  and  the  moon  disturbs  the  equi- 
librium of  the  moveable  parts  of  the  earth's  surface,  so  as  to 
move  the  waters  of  the  sea  towards  the  point  or  meridian 
above  and  below  which  the  moon  culminates.  If  the  waters 
could  move  without  resistance,  the  elevated  parts  of  the  tidal 
wave  would  exactly  coincide  with  the  moon's  meridian,  and 
under  such  conditions  no  consumption  of  vis  viva  could  take 
place.  In  reality,  however,  the  moving  waters  experience 
resistance,  in  consequence  of  which  the  flow  of  the  tidal 
wave  is  delayed,  and  high  water  occurs  in  the  open  sea  on 
the  average  about  2£  hours  after  the  transit  of  the  moon 
through  the  meridian  of  the  place. 

The  waters  of  the  ocean  move  from  west  and  east  towards 
the  meridian  of  the  moon,  and  the  more  elevated  wave  is,  for 
the  reason  above  stated,  always  to  the  east  of  the  moon's  me- 
ridian ;  hence  the  sea  must  press  and  flow  more  powerfully 
from  east  to  west  than  from  west  to  east.  The  ebb  and  flow 
of  the  tidal  wave  therefore  consists  not  only  in  an  alternate 
rising  and  falling  of  the  waters,  but  also  in  a  slow  progressive 
motion  from  east  to  west.  The  tidal  wave  produces  a  gen- 
eral western  current  in  the  ocean. 

This  current  is  opposite  in  direction  to  the  earth's  rota- 
tion, and  therefore  its  friction  against  and  collision  with  the 
bed  and  shores  of  the  ocean  must  offer  everywhere  resistance 
to  the  axial  rotation  of  the  earth,  and  diminish  the  vis  viva  of 
its  motion.  The  earth  here  plays  the  part  of  a  fly-wheel. 
The  moveable  parts  of  its  surface  adhere,  so  to  speak,  to  the 
relatively  fixed  moon,  and  are  dragged  in  a  direction  opposite 
to  that  of  the  earth's  rotation,  in  consequence  of  which,  ac- 
tion takes  place  between  the  solid  and  liquid  parts  of  this  fly- 


THE  TIDAL  WAVE.  293 

wheel,  resistance  is  overcome,  and  the  given  rotatory  effect 
diminished. 

Water-mills  have  been  turned  by  the  action  of  the  tides  ; 
the  effects  produced  by  such  an  arrangement  are  distinguished 
in  a  remarkable  manner  from  those  of  a  mill  turned  by  a 
mountain-stream.  The  one  obtains  the  vis  viva  with  which 
it  works  from  the  earth's  rotation,  the  other  from  the  sun's 
radiation. 

Various  causes  combine  to  incessantly  maintain,  partly  in 
an  undulatory,  partly  in  a  progressive  motion,  the  waters  of 
the  ocean.  Besides  the  influence  of  the  sun  and  the  moon  on 
the  rotating  earth,  mention  must  be  made  of  the  influence  of 
the  movement  of  the  lower  strata  of  the  atmosphere  on  the 
surface  of  the  ocean,  and  of  the  different  temperatures  of  the 
sea  in  various  climates ;  the  configuration  of  the  shores  and 
the  bed  of  the  ocean  likewise  exercise  a  manifold  influence  on 
the  velocity,  direction,  and  extent  of  the  oceanic  currents. 

The  motions  in  our  atmosphere,  as  well  as  those  of  the 
ocean,  presuppose  the  existence  and  consumption  of  vis  viva 
to  overcome  the  continual  resistances,  and  to  prevent  a  state 
of  rest  or  equilibrium.  Generally  speaking,  the  power  neces- 
sary for  the  production  of  aerial  currents  may  be  of  threefold 
origin.  Either  the  radiation  of  the  sun,  the  heat  derived 
from  a  store  in  the  interior  of  the  earth,  or,  lastly,  the  rota- 
tory effect  of  the  earth  may  be  the  source. 

As  far  as  quantity  is  concerned  the  sun  is  by  far  the  most 
important  of  the  above.  According  to  Pouillet's  measure- 
ments, a  square  metre  of  the  earth's  surface  receives  on  the 
average  4*408  units  of  heat  from  the  sun  per  minute.  Since 
one  unit  of  heat  is  equivalent  to  367  Km,  it  follows  that  one 
square  metre  of  the  surface  of  our. globe  receives  per  minute 
an  addition  of  vis  viva  equal  to  1620  Km,  or  the  whole  of  the 
earth's  surface  in  the  same  time  825,000  billions  of  Km.  A 
power  of  75  Km  per  second  is  called  a  horse-power.  Ac- 
cording to  this,  the  effect  of  the  solar  radiation  in  mechanical 


294:  CELESTIAL   DYNAMICS. 

work  on  one  square  metre  of  the  earth's  surface  would  be 
equal  to  0'36,  and  the  total  effect  for  the  whole  globe  180  bil- 
lions of  horse-powers.  A  not  inconsiderable  portion  of  this 
enormous  quantity  of  vis  viva  is  consumed  in  the  production 
of  atmospheric  actions,  in  consequence  of  which  numerous 
motions  are  set  up  in  the  earth's  atmosphere. 

In  spite  of  their  great  variety,  the  atmospheric  currents 
may  be  reduced  to  a  single  type.  In  consequence  of  the  une- 
qual heating  of  the  earth  in  different  degrees  of  latitude,  the 
colder  and  heavier  air  of  the  polar  regions  passes  in  an  under 
current  towards  the  equator  ;  whereas  the  heated  air  of  the 
tropics  ascends  to  the  higher  parts  of  the  atmosphere,  and 
flows  from  thence  towards  the  poles.  In  this  manner  the  air 
of  each  hemisphere  performs  a  circuitous  motion. 

It  is  known  that  these  currents  are  essentially  modified  by 
the  motion  of  the  earth  on  its  axis.  The  polar  currents,  with 
their  smaller  rotatory  velocity,  receive  a  motion  from  east  to 
west  contrary  to  the  earth's  rotation,  and  the  equatorial  cur- 
rents one  from  west  to  east  in  advance  of  the  axial  rotation 
of  the  earth.  The  former  of  these  currents,  the  easterly 
winds,  must  diminish  the  rotatory  effect  of  the  globe,  the  lat- 
ter, the  westerly  winds,  must  increase  the  same  power.  The 
final  result  of  the  action  of  these  opposed  influences  is,  as  re- 
gards the  rotation  of  the  earth,  according  to  well-known  me- 
chanical principles,  =0 ;  for  these  currents  counteract  each 
other,  and  therefore  cannot  exert  the  least  influence  on  the 
axial  rotation  of  the  earth.  This  important  conclusion  was 
proved  by  Laplace. 

The  same  law  holds  good  for  every  imaginable  action 
which  is  caused  either  by  the  radiant  heat  of  the  sun,  or  by 
the  heat  which  reaches  the  surface  from  the  earth's  interior, 
whether  the  action  be  in  the  air,  in  the  water,  or  on  the  land. 
The  effect  of  every  single  motion  produced  by  these  means  on 
the  rotation  of  the  globe,  is  exactly  compensated  by  the  effect 
of  another  motion  in  an  opposite  direction  ;  so  that  the  result- 


THE   TIDAL   WAVE.  295 

ant  of  all  these  motions  is,_as  far  as  the  axial  rotation  of  the 
globe  is  concerned,  =  0. 

In  those  actions  known  as  the  tides,  such  compensation, 
however,  does  not  take  place ;  for  the  pressure  or  pull  by 
which  they  are  produced  is  always  stronger  from  east  to  west 
than  from  west  to  east.  The  currents  caused  by  this  pull 
may  ebb  and  flow  in  different  directions,  but  their  motion  pre- 
dominates in  that  which  is  opposed  to  the  earth's  rotation. 

The  velocity  of  the  currents  caused  by  the  tide  of  the  at- 
mosphere amounts,  according  to  Laplace's  calculation,  to  not 
more  than  75  millimetres  in  a  second,  or  nearly  a  geographi- 
cal mile  in  twenty-four  hours ;  it  is  clear  that  much  more 
powerful  effects  produced  by  the  sun's  heat  would  hide  this 
action  from  observation.  The  influence  of  these  air-currents, 
however,  on  the  rotatory  effect  of  the  earth  is,  according  to 
the  laws  of  mechanics,  exactly  the  same  as  it  would  be  were 
the  atmosphere  undisturbed  by  the  sun's  radiant  heat. 

The  combined  motions  of  air  and  water  are  to  be  regarded 
from  the  same  point  of  view.  If  we  imagine  the  influence 
of  the  sun  and  that  of  the  interior  of  our  globe  not  to  exist, 
the  motion  of  the  air  and  ocean  from  east  to  west  is  still  left 
as  an  obstacle  to  the  axial  rotation  of  the  earth. 

The  motion  of  the  waters  of  the  ocean  from  east  to  west 
was  long  ago  verified  by  observation,  and  it  is  certain  that 
the  tides  are  the  most  effectual  of  the  causes  to  which  this 
great  westerly  current  is  to  be  referred. 

Besides  the  tidal  wave,  the  lower  air-currents  moving  in 
the  same  direction,  the  trade-winds  of  the  tropics  especially, 
may  be  assigned  as  causes  of  this  general  movement  of  the 
waters.  The  westerly  direction  of  the  latter,  however,  is  not 
confined  to  the  region  of  easterly  winds  ;  it  is  met  with  in  the 
region  of  perpetual  calms,  where  it  possesses  a  velocity  of 
several  miles  a  day ;  it  is  observed  far  away  from  the  tropics 
both  north  and  south,  in  regions  where  westerly  winds  pre- 


296  CELESTIAL    DYNAMICS. 

vail,  near  the  Cape  of  Good  Hope,  the  Straits  of  Magellan, 
the  arctic  regions,  &c. 

A  third  cause  for  the  production  of  a  general  motion  of 
translation  of  the  waters  of  the  ocean  is  the  unequal  heating 
of  the  sea  in  different  zones.  According  to  the  laws  of  hy- 
drostatics, the  colder  water  of  the  higher  degrees  of  latitude 
is  compelled  to  flow  towards  the  equator,  and  the  warmer 
water  of  the  tropics  towards  the  poles,  in  consequence  of 
which,  similar  movements  are  produced  in  the  ocean  to  those 
in  the  atmosphere.  This  is  the  cause  of  the  cold  under  cur- 
rent from  the  poles  to  the  equator,  and  of  the  warm  surface- 
current  from  the  equator  to  the  poles.  The  waters  of  the  lat- 
ter, by  virtue  of  the  greater  velocity  of  rotation  at  the  equa- 
tor, assume  in  their  onward  progress  a  direction  from  west  to 
east.  It  is  a  striking  proof  of  the  preponderating  influence 
of  the  tidal  wave  that,  in  spite  of  this,  the  motion  of  the 
ocean  is  on  the  whole  in  an  opposite  direction. 

Theory  and  experience  thus  agree  in  the  result  that  the 
influence  of  the  moon  on  the  rotating  earth  causes  a  motion 
of  translation  from  east  to  west  in  both  atmosphere  and 
ocean.  This  motion  must  continually  diminish  the  rotatory 
effect  of  the  earth,  for  want  of  an  opposite  and  compensating 
influence. 

The  continual  pressure  of  the  tidal  wave  against  the  axial 
rotation  of  the  earth  may  also  be  deduced  from  statical  laws. 

The  gravitation  of  the  moon  affects  without  exception  all 
parts  of  the  globe.  Let  the  earth  be  divided  by  the  plane  of 
the  meridian  in  which  the  moon  happens  to  be  into  two  hemi- 
spheres, one  to  the  east,  the  other  to  the  west  of  this  merid- 
ian. It  is  clear  that  the  moon,  by  its  attraction  of  the  east- 
ern hemisphere,  tends  to  retard  the  motion  of  the  earth,  and 
by  its  attraction  of  the  western  hemisphere,  to  accelerate  the 
same  rotation. 

Under  certain  conditions  both  these  tendencies  compensate 
each  other,  and  then  the  action  of  the  moon  on  the  earth's 


THE  TIDAL  WAVE.  297 

rotation  becomes  zero.  This  happens  when  both  hemispheres 
are  arranged  in  a  certain  manner  symmetrically,  or  when  no 
parts  of  the  earth  can  change  their  relative  position ;  in  the 
latter  case  a  sort  of  symmetry  is  produced  by  the  rotation. 

The  form  of  the  earth  deviates  from  a  perfectly  symmet- 
rical sphere  on  account  of  the  three  following  causes  : — (1) 
the  flattening  of  the  poles,  (2)  the  mountains  on  the  surface, 
and  (3)  the  tidal  wave.  The  first  two  causes  do  not  change 
the  velocity  of  the  earth's  axial  rotation.  In  order  to  com- 
prehend clearly  the  effect  of  the  tidal  wave,  we  shall  imagine 
the  earth  to  be  a  perfectly  symmetrical  sphere  uniformly  sur- 
rounded by  water.  The  attraction  of  the  sun  and  the  moon 
disturbs  the  equilibrium  of  this  mass,  and  two  flat  mountains 
of  water  are  formed.  The  top  of  one  of  these  is  directed 
towards  the  moon,  and  the  summit  of  the  other  is  turned 
away  from  it.  A  straight  line  passing  through  the  tops  of 
these  two  mountains  is  called  the  major  axis  of  this  earth- 
spheroid. 

In  this  state  the  earth  may  be  imagined  to  be  divided 
into  three  parts — a  smaller  sphere,  and  two  spherical  seg- 
ments attached  to  the  opposite  sides  of  the  latter,  and  repre- 
senting the  elevations  of  the  tidal  wave.  The  attraction  of 
the  moon  on  the  small  central  sphere  does  not  change  the  ro- 
tation, and  we  have  therefore  only  to  consider  the  influence 
of  this  attraction  on  the  two  tidal  elevations.  The  upper  ele- 
vation or  mountain,  the  one  nearest  the  moon,  is  attracted 
towards  the  west  because  its  mass  is  principally  situated  to  the 
east  of  the  moon,  and  the  opposite  mountain,  which  is  to  the 
west  of  the  moon,  is  attracted  towards  the  east.  The  upper 
tidal  elevation  is  not  only  more  powerfully  attracted  because 
it  is*  nearer  to  the  moon,  but  also  because  the  angle  under 
which  it  is  pulled  aside  is  more  favourable  for  lateral  deflec- 
tion than  in  the  case  of  the  opposite  protuberance.  The  pres- 
sure from  east  to  west  of  the  upper  elevation  preponderates 
therefore  over  the  pressure  from  west  to  east  of  the  opposite 
13* 


298  CELESTIAL    DYNAMICS. 

mountain  ;  according  to  calculation,  these  quantities  stand  to 
each  other  nearly  as  14  to  13.  From  the  relative  position  of 
these  two  tidal  protuberances  and  the  moon,  or  the  unchange- 
able position  of  the  major  axis  of  the  earth-spheroid  towards 
the  centre  of  gravity  of  the  moon,  a  pressure  results,  which 
preponderates  from  east  to  west,  and  offers  an  obstacle  to  the 
earth's  rotation. 

If  gravitation  were  to  be  compared  with  magnetic  attrac- 
tion, the  earth  might  be  considered  to  be  a  large  magnet,  one 
pole  of  which,  being  more  powerfully  attracted,  would  repre- 
sent the  upper,  and  the  other  pole  the  lower  tidal  elevation. 
As  the  upper  tidal  wave  tends  to  move  towards  the  moon,  the 
earth  would  act  like  a  galvanometer,  whose  needle  has  been 
deflected  from  the  magnetic  meridian,  and  which,  while  tend- 
ing to  return  thereto,  exerts  a  constant  lateral  pressure. 

The  foregoing  discussion  may  suffice  to  demonstrate  the 
influence  of  the  moon  on  the  earth's  rotation.  The  retarding 
pressure  of  the  tidal  wave  may  quantitatively  be  determined 
in  the  same  manner  as  that  employed  in  computing  the  pre- 
cession of  the  equinoxes  and  the  nutation  of  the  earth's  axis. 
The  varied  distribution  of  land  and  water,  the  unequal  and 
unknown  depth  of  the  ocean,  and  the  as  yet  imperfectly  ascer- 
tained mean  difference  between  the  time  of  the  moon's  cul- 
mination and  that  of  high  water  in  the  open  sea,  enter,  how- 
ever, as  elements  into  such  a  calculation,  and  render  the  de- 
sired result  an  uncertain  quantity. 

In  the  mean  time  this  retarding  pressure,  if  imagined  to 
act  at  the  equator,  cannot  be  assumed  to  be  less  than  1000 
millions  of  kilogrammes.  In  order  to  start  with  a  definite 
conception,  we  may  be  allowed  to  use  this  round  number  as  a 
basis  for  the  following  calculations. 

The  rotatory  velocity  of  the  earth  at  the  equator  is  464 
metres,  and  the  consumption  of  mechanical  work,  therefore, 
for  the  maintenance  of  the  tides  464,000  millions  of  Km,  or 
6000  millions  of  horse-powers  per  second.  The  effect  of  the 


THE  TIDAL  WAVE.  299 

tides  may  consequently  be  estimated  at  g^th  of  the  effect  re- 
ceived by  the  earth  from  the  sun. 

The  rotatory  effect  which  the  earth  at  present  possesses, 
may  be  calculated  from  its  mass,  volume,  and  velocity  of  rota- 
tion. The  volume  of  the  earth  is  2,650,686,000  cubic  miles, 
and  its  specific  gravity,  according  to  Reich,  =  5 '44.  If,  for 
the  sake  of  simplicity,  we  assume  the  density  of  the  earth  to 
be  uniform  throughout  its  mass,  we  obtain  from  the  above 
premises,  and  the  known  velocity  of  rotation,  25,840  quadril- 
lions of  kilogrammetres  as  the  rotatory  effect  of  the .  earth. 
If,  during  every  second  in  2500  years,  464,000  millions  of 
Km  of  this  effect  were  consumed  by  the  ebb  and  flow  of  the 
tidal  wave,  it  would  suffer  a  diminution  of  36,600  trillions  of 
Km,  or  about  m^fa  of  its  quantity. 

The  velocities  of  rotation  of  a  sphere  stand  to  each  other 
in  the  same  ratio  as  the  square  roots  of  the  rotatory  effects, 
when  the  volume  of  the  sphere  remains  constant.  From  this 
it  follows  that,  in  the  assumed  time  of  2500  years,  the  length 
of  a  day  has  increased  1>400>000th ;  or  if  a  day  be  taken  equal 
to  86,400  seconds,  it  has  lengthened  ^th  of  a  second,  if  the 
volume  of  the  earth  has  not  changed.  Whether  this  supposi- 
tion be  correct  or  not,  depends  on  the  temperature  of  our 
planet,  and  will  be  discussed  in  the  next  chapter. 

The  tides  also  react  on  the  motion  of  the  moon.  The 
stronger  attraction  of  the  elevation  nearest  to,  and  to  the  east 
of  the  moon,  increases  with  the  tangential  velocity  of  our  sat- 
ellite ;  the  mean  distance  of  the  earth  and  the  moon,  and  the 
time  of  revolution  of  the  latter,  are  consequently  augmented. 
The  effect  of  this  action,  however,  is  insignificant,  and,  ac- 
cording to  calculation,  does  not  amount  to  more  than  a  frac- 
tion of  a  second  in  the  course  of  centuries. 


300  CELESTIAL  DYNAMICS. 


VIII.— THE   EARTH'S   INTERIOR    HEAT. 

WITHOUT  doubt  there  was  once  a  time  when  our  globe 
had  not  assumed  its  present  magnitude.  According  to  this, 
by  aid  of  this  simple  assumption,  the  origin  of  our  planet  may 
be  reduced  to  the  union  of  once  separated  masses. 

To  the  mechanical  combinations  of  masses  of  the  second 
order,  with  masses  of  the  second  and  third  order,  &c.,  the 
same  laws  as  those  enunciated  for  the  sun  apply.  The  collis- 
ion of  such  masses  must  always  generate  an  amount  of  heat 
proportional  to  the  squares  of  their  velocities,  or  to  their  me- 
chanical effect. 

Although  we  are  not  in  a  position  to  affirm  anything  cer- 
tain respecting  the  primordial  conditions  under  which  the 
constituent  parts  of  the  earth  existed,  it  is  nevertheless  of  the 
greatest  interest  to  estimate  the  quantities  of  heat  generated  by 
the  collision  and  combination  of  these  parts  by  a  standard 
based  on  the  simplest  assumptions. 

Accordingly  we  shall  consider  for  the  present  the  earth  to 
have  been  formed  by  the  union  of  two  parts,  which  obtained 
their  relative  motions  by  their  mutual  attraction  only.  Let 
the  whole  mass  of  the  present  earth,  expressed  in  kilo- 
grammes, be  T,  and  the  masses  of  the  two  portions  T—  x  and 
x.  The  ratio  of  these  two  quantities  may  be  imagined  to 
assume  various  values.  The  two  extreme  cases  are,  when  05 
is  considered  infinitely  small  in  comparison  with  T,  and  when 
x  =  T—  x  —  %  T.  These  form  the  limits  of  all  imaginable 
ratios  of  the  parts  T— x  and  a?,  and  will  now  be  more  closely 
examined. 

Terrestrial  heights  are  of  course  excluded  from  the  fol- 
lowing consideration.  In  the  first  place,  let  x,  in  comparison 
with  T  —  as,  be  infinitely  small.  The  final  velocity  with 
which  x  arrives  on  the  surface  of  the  large  mass,  after  having 


THE  EARTH'S  INTERIOR  HEAT.  301 

passed  through  a  great  space  in  a  straight  line,  or  after  pre- 
vious central  motion  round  it,  is,  according  to  the  laws  devel- 
oped in  relation  to  the  sun  in  Chapter  III.,  confined  within 
the  limits  of  7908  and  11,183  metres.  The  heat  generated 
by  this  process  may  amount  to  from  8685  X#  to  17,370  X& 
units,  according  to  the  value  of  the  major  axis  of  the  orbit 
of  x.  This  heat,  however,  vanishes  by  its  distribution  through 
the  greater  mass,  because  x  is,  according  to  supposition,  infi- 
nitely small  in  comparison  with  T. 

The  quantity  of  heat  generated  increases  with  #,  and 
amounts  in  the  second  case,  when  cc  — £T,  to  from  6000 XT 
to  8685 XT  units. 

If  we  assume  the  earth  to  possess  a  very  great  capacity 
for  heat,  equal  in  fact  to  that  of  its  volume  of  water,  which 
when  calculated  for  equal  weights  =  0*184,  the  above  discus- 
sion leads  to  the  conclusion  that  the  difference  of  temperature 
of  the  constituent  parts,  and  of  the  earth  after  their  union,  or, 
in  other  words,  the  heat  generated  by  the  collision  of  these 
parts,  may  range,  according  to  their  relative  magnitude,  from 
0°  to  32,000°,  or  even  to  47,000°  ! 

With  the  number  of  parts  which  thus  mechanically  com- 
bine, the  quantity  of  heat  developed  increases.  Far  greater 
still  would  have  been  the  generation  of  heat  if  the  constituent 
parts  had  moved  in  separate  orbits  round  the  sun  before  their 
union,  and  had  accidentally  approached  and  met  each  other. 
For  various  reasons,  however,  this  latter  supposition  is  not 
very  probable. 

Several  facts  indicate  that  our  earth  was  once  a  fiery 
liquid  mass,  which  has  since  cooled  gradually,  down  to  a  com- 
paratively inconsiderable  depth  from  the  surface,  to  its  pres- 
ent temperature.  The  first  proof  of  this  is  the  form  of  the 
earth.  "  The  form  of  the  earth  is  its  history."  According 
to  the  most  careful  measurements,  the  flattening  at  the  poles 
is  exactly  such  as  a  liquid  mass  rotating  on  its  axis  with  the 
velocity  of  the  earth  would  possess ;  from  this  we  may  con- 


302  CELESTIAL   DYNAMICS. 

elude  that  the  earth  at  the  time  it  received  its  rotatory  motion 
was  in  a  liquid  state  ;  and,  after  much  controversy,  it  may  be 
considered  as  settled  that  this  liquid  condition  was  not  that  of 
an  aqueous  solution,  but  of  a  mass  melted  by  a  high  tempera- 
ture. 

The  temperature  of  the  crust  of  the  globe  likewise  fur- 
nishes proof  of  the  existence  of  a  store  of  heat  in  its  interior. 
Many  exact  experiments  and  measurements  show  that  the 
temperature  of  the  earth  increases  with  the  depth  to  which 
we  penetrate.  In  boring  the  artesian  well  at  Grenelle,  which 
is  546  metres  deep,  it  was  observed  that  the  temperature  aug- 
mented at  the  rate  of  1°  for  every  30  metres.  The  same  re- 
sult was  obtained  by  observations  in  the  artesian  well  at  Mon- 
dorf  in  Luxembourg :  this  well  is  671  metres  in  depth,  and 
its  water  34°  warm. 

Thermal  springs  furnish  a  striking  proof  of  the  high  tem- 
perature existing  in  the  interior  of  the  earth.  Scientific  men 
are  agreed  that  the  aqueous  deposits  from  the  atmosphere, 
rain,  hail,  dew,  and  snow,  are  the  sole  causes  of  the  forma- 
tion of  springs.  The  water  obeying  the  laws  of  gravity,  per- 
colates through  the  earth  wherever  it  can,  and  reappears  at 
the  surface  in  places  of  a  lower  situation.  When  water  sinks 
to  considerable  depths  through  vertical  crevices  in  the  rocks, 
it  acquires  the  temperature  of  the  surrounding  strata,  and 
returns  as  a  thermal  spring  to  the  surface. 

Such  waters  are  frequently  distinguished  from  the  water 
of  ordinary  springs  merely  by  their  possessing  a  higher  tem- 
perature. If,  however,  the  water  in  its  course  meets  with 
mineral  or  organic  substances  which  it  can  dissolve  and  re- 
tain, it  then  reappears  as  a  mineral  spring.  Examples  of 
such  are  met  with  at  Aachen,  Carlsbad,  &c. 

In  a  far  more  decided  manner  than  by  the  high  tempera- 
ture of  the  water  of  certain  springs,  the  interior  heat  of  our 
globe  is  made  manifest  by  those  fiery  fluid  masses  which 
sometimes  rise  from  considerable  depths.  The  temperature 


THE  EARTH'S  INTERIOR  HEAT.  303 

of  the  earth's  crust  increases  at  the  rate  of  1°  for  every  30 
metres  we  descend  from  the  surface  towards  the  centre.  Al- 
though it  is  incredible  that  this  augmentation  can  continue  at 
the  same  rate  till  the  centre  be  reached,  we  may  nevertheless 
assume  with  certainty  that  it  does  continue  to  a  considerable 
depth.  Calculation  based  on  this  assumption  shows  that  at 
a  depth  of  a  few  miles  a  temperature  must  exist  sufficiently 
powerful  to  fuse  most  substances.  Such  molten  masses  pene- 
trate the  cold  crust  of  the  globe  in  many  places,  and  make 
their  appearance  as  lava. 

A  distinguished  scientific  man  has  lately  expressed  him- 
self on  the  origin  of  the  interior  heat  of  the  earth  as  follows : 
— "  No  one  of  course  can  explain  the  final  causes  of  things. 
This  much,  however,  is  clear  to  every  thinking  man,  that 
there  is  just  as  much  reason  that  a  body,  like  the  earth,  for 
example,  should  be  warm,  warmer  than  ice  or  human  blood, 
as  there  is  that  it  should  be  cold  or  colder  than  the  latter.  A 
particular  cause  for  this  absolute  heat  is  as  little  necessary  as 
a  cause  for  motion  or  rest.  Change — that  is  to  say,  transi- 
tion from  one  state  of  things  to  another — alone  requires  and 
admits  of  explanation." 

It  is  evident  that  this  reflection  is  not  fitted  to  suppress  the 
desire  for  an  explanation  of  the  phenomenon  in  question.  As 
all  matter  has  the  tendency  to  assume  the  same  temperature 
as  that  possessed  by  the  substances  by  which  it  happens  to  be 
surrounded,  and  to  remain  in  a  quiescent  state  as  soon  as 
equilibrium  has  been  established,  we  must  conclude  that, 
whenever  we  meet  with  a  body  warmer  than  its  neighbours, 
such  body  must  have  received  at  a  (relatively  speaking)  not 
far  distant  time,  a  certain  degree  of  heat, — a  process  which 
certainly  allows  of,  and  requires  explanation. 

Newton's  theory  of  gravitation,  whilst  it  enables  us  to  de- 
termine, from  its  present  form,  the  earth's  state  of  aggrega- 
tion in  ages  past,  at  the  same  time  points  out  to  us  a  source 
of  heat  powerful  enough  to  produce  such  a  state  of  aggrega- 


304  CELESTIAL  DYNAMICS. 

tion,  powerful  enough  to  melt  worlds ;  it  teaches  us  to  con- 
sider the  molten  state  of  a  planet  as  the  result  of  the  mechan- 
ical union  of  cosmical  masses,  and  thus  derive  the  radiation 
of  the  sun  and  the  heat  in  the  bowels  of  the  earth  from  a 
common  origin. 

The  rotatory  effect  of  the  earth  also  may  be  readily  ex- 
plained by  the  collision  of  its  constituent  parts  ;  and  we  must 
accordingly  subtract  the  vis  viva  of  the  axial  rotation  from 
the  whole  effect  of  the  collision  and  mechanical  combination, 
in  order  to  obtain  the  quantity  of  heat  generated.  The  rota- 
tory effect,  however,  is  only  a  small  quantity  in  comparison 
with  the  interior  heat  of  the  earth.  It  amounts  to  about 
4400  X  T  kilogrammetres,  T  being  the  weight  of  the  earth  in 
kilogrammes,  which  is  equivalent  to  12 XT  units  of  heat,  if 
we  assume  the  density  of  the  earth  to  be  uniform  throughout. 

If  we  imagine  the  moon  in  the  course  of  time,  either  in 
consequence  of  the  action  of  a  resisting  medium  or  from 
some  other  cause,  to  unite  herself  with  our  earth,  two  princi- 
pal effects  are  to  be  discerned.  A  result  of  the  collision 
would  be,  that  the  whole  mass  of  the  moon  and  the  cold  crust 
of  the  earth  would  be  raised  some  thousands  of  degrees  in 
temperature,  and  consequently  the  surface  of  the  earth  would 
be  converted  into  a  fiery  ocean.  At  the  same  time  the  velo- 
city of  the  earth's  axial  rotation  would  be  somewhat  acceler- 
ated, and  the  position  of  its  axis  with  regard  to  the  heavens, 
and  to  its  own  surface,  slightly  altered.  If  the  earth  had 
been  a  cold  body  without  axial  rotation,  the  process  of  its 
combining  with  the  moon  would  have  imparted  to  it  both 
heat  and  rotation. 

It  is  probable  that  such  processes  of  combination  between 
different  parts  of  our  globe  may  have  repeatedly  happened 
before  the  earth  attained  its  present  magnitude,  and  that  lux- 
uriant vegetation  may  have  at  different  times  been  buried  un- 
der the  fiery  debris  resulting  from  the  conflict  of  these  masses. 


THE  EARTH'S  mTERiOR  HEAT.  305 

As  long  as  the  surface  of  our  globe  was  in  an  incandes- 
cent state,  it  must  have  lost  heat  at  a  very  .rapid  rate  ;  grad- 
ually this  process  became  slower  ;  and  although  it  has  not  yet 
entirely  ceased,  the  rate  of  cooling  must  have  diminished  to  a 
comparatively  small  magnitude. 

Two  phenomena  are  caused  by  the  cooling  of  the  earth, 
which,  on  account  of  their  common  origin,  are  intimately  re- 
lated. The  decrease  of  temperature,  and  consequent  contrac- 
tion of  the  earth's  crust,  must  have  caused  frequent -distur- 
bances and  revolutions  on  its  surface,  accompanied  by  the 
ejection  of  molten  masses  and  the  formation  of  protuberances  ; 
on  the  other  hand,  according  to  the  laws  of  mechanics,  the 
velocity  of  rotation  must  have  increased  with  the  diminution 
of  the  volume  of  the  sphere,  or,  in  other  words,  the  cooling 
of  the  earth  must  have  shortened  the  length  of  the  day. 

As  the  intensity  of  such  disturbances  and  the  velocity  of 
rotation  are  closely  connected,  it  is  clear  that  the  youth  of  our 
planet  must  have  been  distinguished  by  continual  violent 
transformations  of  its  crust,  and  a  perceptible  acceleration  of 
the  velocity  of  its  axial  rotation ;  whilst  in  the  present  time 
the  metamorphoses  of  its  surface  are  much  slower,  and  the 
acceleration  of  its  axial  revolution  diminished  to  a  very  small 
amount. 

If  we  imagine  the  times  when  the  Alps,  the  chain  of  the 
Andes,  and  the  Peak  of  Teneriffe  were  upheaved  from  the 
deep,  and  compare  with  such  changes  the  earthquakes  and 
volcanic  eruptions  of  historic  times,  we  perceive  in  these 
modern  transformations  but  weak  images  of  the  analogous 
processes  of  bygone  ages. 

Whilst  we  are  surrounded  on  every  side  by  the  monu- 
ments of  violent  volcanic  convulsions,  we  possess  no  record 
of  the  velocity  of  the  axial  rotation  of  our  planet  in  antedi- 
luvian times.  It  is  of  the  greatest  importance  that  we  should 
have  an  exact  knowledge  of  a  change  in  this  velocity,  or  in 
the  length  of  the  day  during  historic  times.  The  investiga- 


306  t        CELESTIAL  DYNAMICS. 

tion  of  this  subject  by  the  great  Laplace  forms  a  bright  mon- 
ument in  the  department  of  exact  science. 

These  calculations  are  essentially  conducted  in  the  follow- 
ing manner : — In  the  first  place,  the  time  between  two  eclipses 
of  the  sun,  widely  apart  from  each  other,  is  as  accurately 
as  possible  expressed  in  days,  and  from  this  the  ratio  of  the 
time  of  the  earth's  rotation  to  the  mean  time  of  the  moon's 
revolution  determined.  If,  now,  the  observations  of  ancient 
astronomers  be  compared  with  those  of  our  present  time,  the 
least  alteration  in  the  absolute  length  of  a  day  may  be  de- 
tected by  a  change  in  this  ratio,  or  in  a  disturbance  in  the 
lunar  revolution.  The  most  perfect  agreement  of  ancient  rec- 
ords on  the  movements  of  the  moon  and  the  planets,  on  the 
eclipses  of  the  sun,  &c.,  revealed  to  Laplace  the  remarkable 
fact  that  in  the  course  of  25  centuries,  the  time  in  which  our 
earth  revolves  on  its  axis  has  not  altered  g^th  part  of  a  sexa- 
gesimal second ;  and  the  length  of  a  day  therefore  may  be 
considered  to  have  been  constant  during  historic  times. 

This  result,  as  important  as  it  was  convenient  for  astron- 
omy, was  nevertheless  of  a  nature  to  create  some  difficulties 
for  the  physicist.  With  apparently  good  reason  it  was  con- 
cluded that,  if  the  velocity  of  rotation  had  remained  constant, 
the  volume  of  the  earth  could  have  undergone  no  change. 
The  earth  completes  one  revolution  on  its  axis  in  86,400  si- 
dereal seconds  ;  it  consequently  appears,  if  this  time  has  not 
altered  during  2500  years  to  the  extent  of  ^th  of  a  second, 
or  43>000>000th  part  of  a  day,  that  during  this  long  space  of  time 
the  radius  of  the  earth  also  cannot  have  altered  more  than 
this  fraction  of  its  length.  The  earth's  radius  measures 
6,369,800  metres,  and  therefore  its  length  ought  not  to  have 
diminished  more  than  15  centimetres  in  25  centuries. 

The  diminution  in  volume,  as  a  result  of  the  cooling-pro- 
cess, is,  however,  closely  connected  with  the  changes  on  the 
earth's  surface.  When  we  consider  that  scarcely  a  day 
passes  without  the  occurrence  of  an  earthquake  or  shock  in 


THE  EARTH'S  INTERIOR  HEAT.  307 

one  place  or  another,  and  that  of  the  300  active  volcanos 
some  are  always  in  action,  it  would  appear  that  such  a  lively 
reaction  of  the  interior  of  the  earth  against  the  crust  is  in- 
compatible with  the  constancy  of  its  volume. 

This  apparent  discrepancy  between  Cordier's  theory  of  the 
connexion  between  the  cooling  of  the  earth  and  the  reaction 
of  the  interior  on  the  exterior  parts,  and  Laplace's  calcula- 
tion showing  the  constancy  of  the  length  of  the  day,  a  calcu- 
lation which  is  undoubtedly  correct,  has  induced  most  scien- 
tific men  to  abandon  Cordier's  theory,  and  thus  to  deprive 
themselves  of  any  tenable  explanation  of  volcanic  activity. 

The  continued  cooling  of  the  earth  cannot  be  denied,  for 
it  takes  place  according  to  the  laws  of  nature ;  in  this  respect 
the  earth  cannot  comport  itself  differently  from  any  other 
mass,  however  small  it  may  be.  In  spite  of  the  heat  which 
it  receives  from  the  sun,  the  earth  will  have  a  tendency  to 
cool  so  long  as  the  temperature  of  its  interior  is  higher  than 
the  mean  temperature  of  its  surface.  Between  the  tropics  the 
mean  temperature  produced  by  the  sun  is  about  28°,  and  the 
sun  therefore  is  as  little  able  to  stop  the  cooling-tendency  of 
the  earth  as  the  moderate  warmth  of  the  air  can  prevent  the 
cooling  of  a  red-hot  ball  suspended  in  a  room. 

Many  phenomena,  for  instance  the  melting  of  the  glaciers 
near  the  bed  on  which  they  rest,  show  the  uninterrupted 
emission  of  heat  from  the  interior  towards  the  exterior  of  the 
earth ;  and  the  question  is,  Has  the  earth  in  25  centuries 
actually  lost  no  more  heat  than  that  which  is  requisite  to 
shorten  a  radius  of  more  than  6  millions  of  metres  only  15 
centimetres  ? 

In  answering  this  question,  three  points  enter  into  our 
calculation ; — (1)  the  absolute  amount  of  heat  lost  by  the 
earth  in  a  certain  time,  say  one  day ;  (2)  the  earth's  capacity 
for  heat ;  and  (3)  the  coefficient  of  expansion  of  the  mass  of 
the  earth*. 

As  none  of  these  quantities  can  be  determined  by  direct 


308  CELESTIAL  DYNAMICS. 

measurements,  we  are  obliged  to  content  ourselves  with  prob- 
able estimates ;  these  estimates  will  carry  the  more  weight 
the  less  they  are  formed  in  favour  of  some  preconceived  opin- 
ion. 

Considering  what  is  known  about  the  expansion  and  con- 
traction of  solids  and  liquids  by  heat  and  cold,  we  arrive  at 
the  conclusion  that  for  a  diminution  of  1°  in  temperature, 
the  linear  contraction  of  the  earth  cannot  well  be  less  than 
ioo^o6tft  part,  a  number  which  we  all  the  more  readily  adopt 
because  it  has  been  used  by  Laplace,  Arago,  and  others. 

If  we  compare  the  capacity  for  heat  of  all  solid  and  liquid 
bodies  which  have  been  examined,  we  find  that,  both  as  re- 
gards volume  and  weight,  the  capacity  of  water  is  the  great- 
est. Even  the  gases  come  under  this  rule ;  hydrogen,  how- 
ever, forms  an  exception,  it  having  the  greatest  capacity  for 
heat  of  all  bodies  when  compared  with  an  equal  weight  of 
water.  In  order  not  to  take  the  capacity  for  heat  of  the  mass 
of  the  earth  too  small,  we  shall  consider  it  to  be  equal  to  that 
of  its  volume  of  water,  which,  when  calculated  for  equal 
weights,  amounts  to  0*184.* 

If  we  accept  Laplace's  result,  that  the  length  of  a  day  has 
remained  constant  during  the  last  2500  years,  and  conclude 

*  The  capacity  for  heat,  as  well  as  the  coefficient  of  expansion  of  mat- 
ter, as  a  rule,  increases  at  higher  temperatures.  As,  however,  these  two 
quantities  act  in  opposite  ways  in  our  calculations,  we  may  be  allowed  to 
dispense  with  the  influence  which  the  high  temperature  of  the  interior  of 
the  earth  must  exercise  on  these  numbers.  Even  if,  in  consequence  of 
the  high  temperature  of  the  interior,  the  earth's  mass  could  have  a  capa- 
city two  or  three  tunes  as  great  as  that  which  it  has  from  0°  to  100°,  it 
is  to  be  considered,  on  the  other  hand,  that  the  coefficient  of  expansion, 
-.^-i--^  only  holds  good  for  solids,  and  is  even  small  for  them,  whilst  in 
the  case  of  liquids  we  have  to  assume  a  much  greater  coefficient :  for  mer- 
cury between  0°  and  100°,  it  is  about  six  times  as  great.  Especially  great 
is  the  contraction  and  expansion  of  bodies  when  they  change  their  state 
of  aggregation ;  and  this  should  be  taken  into  account  when  considering 
the  formation  of  the  earth's  crust. 


THE  EARTH'S  INTERIOR  HEAT.  309 

that  the  earth's  radius  has  not  diminished  1£  decimetre  in 
consequence  of  cooling,  we  are  obliged  to  assume,  according 
to  the  premises  stated,  that  the  mean  temperature  of  our 
planet  cannot  have  decreased  ^°  in  the  same  period  of  time. 

The  volume  of  the  earth  amounts  to  2650  millions  of  cu- 
bic miles.  A  loss  of  heat  sufficient  to  cool  this  mass  555° 
would  be  equal  to  the  heat  given  off  when  the  temperature 
of  6,150,000  cubic  miles  of  water  decreases  1°  ;  hence  the 
loss  for  one  day  would  be  equal  to  6*74  cubic  miles  of  heat. 

Fourier  has  investigated  the  loss  of  heat  sustained  by  the 
earth.  Taking  the  observation  that  the  temperature  of  the 
earth  increases  at  the  rate  of  1°  for  every  30  metres  as  the 
basis  of  his  calculations,  this  celebrated  mathematician  finds 
the  heat  which  the  globe  loses  by  conduction  through  its  crust 
in  the  space  of  100  years  to  be  capable  of  melting  a  layer  of 
ice  3  metres  in  thickness  and  covering  the  whole  surface  of 
the  globe  ;  this  corresponds  in  one  day  to  7*7  cubic  miles  of 
heat,  and  in  2500  years  to  a  decrease  of  17  centimetres  in 
the  length  of  the  radius. 

According  to  this,  the  cooling  of  the  globe  would  be  suffi- 
ciently great  to  require  attention  when  the  earth's  velocity  of 
rotation  is  considered. 

At  the  same  time  it  is  clear  that  the  method  employed  by 
Fourier  can  only  bring  to  our  knowledge  one  part  of  the  heat 
which  is  annually  lost  by  the  earth ;  for  simple  conduction 
through  terra  firma  is  not  the 'only  way  by  which  heat  escapes 
from  our  globe. 

In  the  first  place,  we  may  make  mention  of  the  aqueous 
deposits  of  our  atmosphere,  which,  as  far  as  they  penetrate 
our  earth,  wash  away,  so  to  speak,  a  portion  of  the  heat,  and 
thus  accelerate  the  cooling  of  the  globe.  The  whole  quantity 
of  water  which  falls  fisom  the  atmosphere  upon  the  land  in 
one  day,  however,  cannot  be  assumed  to  be  much  more  than 
half  a  cubic  mile  in  volume,  hence  the  cooling  effect  produced 
by  this  water  may  be  neglected  in  our  calculation.  The  heat 


310  CELESTIAL   DYNAMICS. 

carried  off  by  all  the  thermal  springs  in  the  world  is  very 
small  in  comparison  with  the  quantities  which  we  have  to 
consider  here. 

Much  more  important  is  the  effect  produced  by  active  vol- 
canos.  As  the  heat  which  accompanies  the  molten  matter  to 
the  surface  is  derived  from  the  store  in  the  interior  of  the 
earth,  their  action  must  influence  considerably  the  diminution 
of  the  earth's  heat.  And  we  have  not  only  to  consider  here 
actual  eruptions  which  take  place  in  succession  or  simulta- 
neously at  different  parts  of  the  earth's  surface,  but  also  vol- 
canos  in  a  quiescent  state,  which  continually  radiate  large 
quantities  of  heat  abstracted  from  the  interior  of  the  globe. 
If  we  compare  the  earth  to  an  animal  body,  we  may  regard 
each  volcano  as  a  place  where  the  epidermis  has  been  torn 
off,  leaving  the  interior  exposed,  and  thus  opening  a  door  for 
the  escape  of  heat. 

Of  the  whole  of  the  heat  which  passes  away  through 
these  numerous  outlets,  too  low  an  estimate  must  not  be 
made.  To  have  some  basis  for  the  estimation  of  this  loss, 
we  have  to  recollect  that  in  1783  Skaptar-Jokul,  a  volcano  in 
Iceland,  emitted  sufficient  lava  in  the  space  of  six  weeks  to 
cover  60  square  miles  of  country  to  an  average  depth  of  200 
metres,  or,  in  other  words,  about  1J  cubic  miles  of  lava. 
The  amount  of  heat  lost  by  this  one  eruption  of  one  volcano 
must,  when  the  high  temperature  of  the  lava  is  considered, 
be  estimated  to  be  more  than  1000  cubic  miles  of  heat ;  and 
the  whole  loss  resulting  from  the  action  of  all  the  volcanos 
amounts,  therefore,  in  all  probability,  to  thousands  of  cubic 
miles  of  heat  per  annum.  This  latter  number,  when  added 
to  Fourier's  result,  produces  a  sum  which  evidently  does  not 
agree  with  the  assumption  that  the  volume  of  our  earth  has 
remained  unchanged. 

In  the  investigation  of  the  cooling  of  our  globe,  the  influ- 
ence of  the  water  of  the  ocean  has  to  be  taken  into  account. 
Fourier's  calculations  are  based  on  the  observations  of  the  in- 


311 

crease  of  the  temperature  of  the  crust  of  our  earth,  from  the 
surface  towards  the  centre.  But  two-thirds  of  the  surface  of 
our  globe  are  covered  with  water,  and  we  cannot  assume  a 
priori  that  this  large  area  loses  heat  at  the  same  rate  as  the 
solid  parts ;  on  the  contrary,  various  circumstances  indicate 
that  the  cooling  of  our  globe  proceeds  more  quickly  through 
the  waters  of  the  ocean  resting  on  it  than  from  the  solid  parts 
merely  in  contact  with  the  atmosphere. 

In  the  first  place,  we  have  to  remark  that  the  bottom  of 
the  ocean  is,  generally  speaking,  nearer  to  the  store  of  heat 
in  the  interior  of  the  earth  than  the  dry  land  is,  and  hence 
that  the  temperature  increases  most  probably  in  a  greater 
ratio  from  the  bottom  of  the  sea  towards  the  interior  of  the 
globe,  than  it  does  in  our  observations  on  the  land.  Sec- 
ondly, we  have  to  consider  that  the  whole  bottom  of  the  sea 
is  covered  by  a  layer  of  ice-cold  water,  which  moves  con- 
stantly from  the  poles  to  the  equator,  and  which,  in  its  pas- 
sage over  sand-banks,  causes,  as  Humboldt  aptly  remarks, 
the  low  temperatures  which  are  generally  observed  in  shallow 
places.  That  the  water  near  the  bottom  of  the  sea,  on  ac- 
count of  its  great  specific  heat  and  its  low  temperature,  is 
better  fitted  than  the  atmosphere  to  withdraw  the  heat  from 
the  earth,  is  a  point  which  requires  no  further  discussion. 

We  have  plenty  of  observations  which  prove  that  the 
earth  suffers  a  great  loss  of  heat  through  the  waters  of  the 
ocean.  Many  investigations  have  demonstrated  the  existence 
of  a  large  expanse  of  sea,  much  visited  by  whalers,  situated 
between  Iceland,  Greenland,  Norway,  and  Spitzbergen,  and 
extending  from  lat.  76°  to  80°  N.,  and  from  long.  15°  E.  to 
15°  W.  of  Greenwich,  where  the  temperature  was  observed 
to  be  higher  in  the  deeper  water  than  near  the  surface — an 
experience  which  neither  accords  with  the  general  rule,  nor 
agrees  with  the  laws  of  hydrostatics.  Franklin  observed,  in 
lat.  77°  N.  and  long.  12°  E.,  that  the  temperature  of  the  sea 
near  the  surface  was  —  £°,  and  at  a  depth  of  700  fathoms 


312  CELESTIAL  DYNAMICS. 

+  6°.  Fisher,  in  lat.  80°  N.  and  long.  11°  E.,  noticed  that 
the  surface-water  had  a  temperature  of  0°,  whilst  at  a  depth 
of  140  fathoms  it  stood  at-f  8°. 

As  sea-water,  unlike  pure  water,  does  not  possess  a  point 
of  greatest  density  at  some  distance  above  the  freezing-point, 
and  as  the  water  in  lat.  80°  N.  is  found  at  some  depth  to  be 
warmer  than  water  at  the  same  depth  10°  southward,  we  can 
only  explain  this  remarkable  phenomenon  of  an  increase  of 
temperature  with  an  increase  of  depth  by  the  existence  of  a 
source  of  heat  at  the  bottom  of  the  sea.  The  heat,  however, 
which  is  required  to  warm  the  water  at  the  bottom  of  an  ex- 
panse of  ocean  more  than  1000  square  miles  in  extent  to  a 
sensible  degree,  must  amount,  according  to  the  lowest  esti- 
mate, to  some  cubic  miles  of  heat  a  day. 

The  same  phenomenon  has  been  observed  in  other  parts 
of  the  world,  such  as  the  west  coast  of  Australia,  the  Adri- 
atic, the  Lago  Maggiore,  &c.  Especial  mention  should  here 
be  made  of  an  observation  by  Horner,  according  to  whom  the 
lead,  when  hauled  up  from  a  depth  varying  from  80  to  100 
fathoms  in  the  mighty  Gulf-stream  off  the  coast  of  America, 
used  to  be  hotter  than  boiling  water. 

The  facts  above  mentioned,  and  some  others  which  might 
be  added,  clearly  show  that  the  loss  of  heat  suffered  by  our 
globe  during  the  last  2500  years  is  far  too  great  to  have  been 
without  sensible  effect  on  the  velocity  of  the  earth's  rotation. 
The  reason  why,  in  spite  of  this  accelerating  cause,  the  length 
of  a  day  has  nevertheless  remained  constant  since  the  most 
ancient  times,  must  be  attributed  to  an  opposite  retarding  ac- 
tion. This  consists  in  the  attraction  of  the  sun  and  moon  on 
the  liquid  parts  of  the  earth's  surface,  as  explained  in  the 
last  chapter. 

According  to  the  calculations  of  the  last  chapter,  the  re- 
tarding pressure  of  the  tides  against  the  earth's  rotation 
would  cause,  during,  the  lapse  of  2500  years,  a  sidereal  day 
to  be  lengthened  to  the  extent  of  f6th  of  a  second ;  as  the 


313 

length  of  a  day,  however,  has  remained  constant,  the  cooling 
effect  of  the  earth  during  the  same  period  of  time  must  have 
shortened  the  day  ^th  of  a  second.  A  diminution  of  the 
earth's  radius  to  the  amount  of  4£  metres  in  2500  years,  and 
a  daily  loss  of  200  cubic  miles  of  heat,  correspond  to  this 
effect.  Hence,  in  the  course  of  the  last  25  centuries,  the 
temperature  of  the  whole  mass  of  the  earth  must  have  de- 
creased {4°. 

The  not  inconsiderable  contraction  of  the  earth  resulting 
from  such  a  loss  of  heat,  agrees  with  the  continual  transfor- 
mations of  the  earth's  surface  by  earthquakes  and  volcanic 
eruptions ;  and  we  agree  with  Cordier,  the  industrious  ob- 
server of  volcanic  processes,  in  considering  these  phenomena 
a  necessary  consequence  of  the  continual  cooling  of  an  earth 
which  is  still  in  a  molten  state  in  its  interior. 

When  our  earth  was  in  its  youth,  its  velocity  of  rotation 
must  have  increased  to  a  very  sensible  degree,  on  account  of 
the  rapid  cooling  of  its  then  very  hot  mass.  This  accelera- 
ting cause  gradually  diminished,  and  as  the  retarding  pressure 
of  the  tidal  wave  remains  nearly  constant,  the  latter  must 
finally  preponderate,  and  the  velocity  of  rotation  therefore 
continually  decrease.  Between  these  two  states  we  have  a 
period  of  equilibrium,  a  period  when  the  influence  of  the 
cooling  and  that  of  the  tidal  pressure  counterbalance  each 
other ;  the  whole  life  of  the  earth  therefore  may  be  divided 
into  three  periods — youth  with  increasing,  middle  age  with 
uniform,  and  old  age  with  decreasing  velocity  of  rotation. 

The  time  during  which  the  two  opposed  influences  on  the 
rotation  of  the  earth  are  in  equilibrium  can,  strictly  speak- 
ing, only  be  very  short,  inasmuch  as  in  one  moment  the  cool- 
ing, and  in  the  next  moment  the  pressure  of  the  tides  must 
prevail.  In  a  physical  sense,  however,  when  measured  by 
human  standards,  the  influence  of  the  cooling,  and  still  more 
so  that  of  the  tidal  wave,  may  for  ages  be  considered  con- 
stant, and  there  must  consequently  exist  a  period  of  many 
14 


314  CELESTIAL  DYNAMICS. 

thousand  years'  duration  during  which  these  counteracting 
influences  will  appear  to  be  equal.  Within  this  period  a  si- 
dereal day  attains  its  shortest  length,  and  the  velocity  of  the 
earth's  rotation  its  maximum — circumstances  which,  accord- 
ing to  mathematical  analysis,  would  tend  to  lengthen  the  du- 
ration of  this  period  of  the  earth's  existence. 

The  historical  times  of  mankind  are,  according  to  La- 
place's calculation,  to  be  placed  in  this  period.  Whether  we 
are  at  the  present  moment  still  near  its  commencement,  its 
middle,  or  are  approaching  its  conclusion,  is  a  question  which 
cannot  be  solved  by  our  present  data,  and  must  be  left  to  fu- 
ture generations. 

The  continual  cooling  of  the  earth  cannot  be  without  an 
influence  on  the  temperature  of  its  surface,  and  consequently 
on  the  climate ;  scientific  men,  led  by  Buffon,  in  fact,  have 
advanced  the  supposition  that  the  loss  of  heat  sustained  by 
our  globe  must  at  some  time  render  it  an  unfit  habitation  for 
organic  life.  Such  an  apprehension  has  evidently  no  founda- 
tion, for  the  warmth  of  the  earth's  surface  is  even  now  much 
more  dependent  on  the  rays  of  the  sun  than  on  the  heat  which 
reaches  us  from  the  interior.  According  to  Pouillet's  meas- 
urements, mentioned  in  Chapter  III.,  the  earth  receives  8000 
cubic  miles  of  heat  a  day  from  the  sun,  whereas  the  heat 
which  reaches  the  surface  from  the  earth's  interior  may  be 
estimated  at  200  cubic  miles  per  diem.  The  heat  therefore 
obtained  from  the  latter  source  every  day  is  but  small  in  com- 
parison to  the  diurnal  heat  received  from  the  sun. 

If  we  imagine  the  solar  radiation  to  be  constant,  and  the 
heat  we  receive  from  the  store  in  the  interior  of  the  earth  to 
be  cut  off,  we  should  have  as  a  consequence  various  changes 
in  the  physical  constitution  of  the  surface  of  our  globe.  The 
temperature  of  hot  springs  would  gradually  sink  down  to  the 
mean  temperature  of  the  earth's  crust,  volcanic  eruptions 
would  cease,  earthquakes  would  no  longer  be  felt,  and  the 
temperature  of  the  water  of  the  ocean  would  be  sensibly  al- 


315 

tered  in  many  places — circumstances  which  would  doubtless 
affect  the  climate  in  many  parts  of  the  world.  Especially  it 
may  be  presumed  that  Western  Europe,  with  its  present  fa- 
vourable climate,  would  become  colder,  and  thus  perhaps  the 
seat  of  the  power  and  culture  of  our  race  transferred  to  the 
milder  parts  of  North  America. 

Be  this  as  it  may,  for  thousands  of  years  to  come  we  can 
predict  no  diminution  of  the  temperature  of  the  surface  of 
our  globe  as  a  consequence  of  the  cooling  of  its  interior  mass  ; 
and,  as  far  as  historic  records  teach,  the  climates,  the  tempe- 
ratures of  thermal  springs,  and  the  intensity  and  frequency 
of  volcanic  eruptions  are  now  the  same  as  they  were  in  the 
far  past. 

It  was  different  in  prehistoric  times,  when  for  centuries 
the  earth's  surface  was  heated  by  internal  fire,  when  mam- 
moths lived  in  the  now  uninhabitable  polar  regions,  and  when 
the  tree-ferns  and  the  tropical  shell-fish  whose  fossil  remains 
are  now  especially  preserved  in  the  coal-formation  were  at 
home  in  all  parts  of  the  world. 


III. 


THE  MECHANICAL  EQUIVALENT  OF  HEAT. 


vast  and  magnificent  structure  of  the  experimental 
_1_  sciences  has  been  erected  on  only  a  few  pillars.  His- 
tory teaches  us  that  the  searching  spirit  of  man  required 
thousands  of  years  for  the  discovery  of  the  fundamental  prin- 
ciples of  the  sciences,  on  which  the  superstructure  was  then 
raised  in  a  comparatively  short  time.  But  these  very  funda- 
mental propositions  are  nevertheless  so  clear  and  simple,  that 
the  discovery  of  them  reminds  us,  in  more  than  one  respect, 
of  Columbus's  egg. 

But  if,  now  that  we  are  at  last  in  possession  of  the  truth, 
we  speak  of  a  method  by  the  application  of  which  the  most 
essential  fundamental  laws  might  have  been  discovered  with- 
out waste  of  time,  it  is  not  that  we  would  criticize  in  any  light 
spirit  the  efforts  and  achievements  of  our  forerunners :  it  is 
merely  with  the  object  of  laying  before  the  reader  in  an  ad- 
vantageous form  one  of  the  additions  to  our  knowledge  which 
recent  times  have  brought  forth. 

The  most  important — not  to  say  the  only — rule  for  the 
genuine  investigation  of  nature  is,  to  remain  firm  in  the  con- 
viction that  the  problem  before  us  is  to  learn  to  know  phenom- 
ena, before  seeking  for  explanations  or  inquiring  after  higher 


TRUE   NATURE   OF   SCIENTIFIC   PROBLEMS.  317 

causes.  As  soon  as  a  fact  is  once  known  in  all  its  relations, 
it  is  therein  explained,  and  the  problem  of  science  is  at  an 
end. 

Notwithstanding  that  some  may  pronounce  this  a  trite 
assertion,  and  no  matter  how  many  arguments  others  may 
bring  to  oppose  it,  it  remains  none  the  less  certain  that  this 
primary  rule  has  been  too  often  disregarded  even  up  to  the 
most  modern  times ;  while  all  the  speculative  operations  of 
even  the  most  highly  gifted  minds  which,  instead  of  taking 
firm  hold  of  facts  as  such,  have  striven  to  rise  above  them, 
have  as  yet  borne  but  barren  fruit. 

We  shall  not  here  discuss  the  modern  naturalistic  philoso- 
phy (Naturphilosophie)  further  than  to  say  that  its  character 
is  already  sufficiently  apparent  from  the  ephemeral  existence 
of  its  offspring.  But  even  the  greatest  and  most  meritorious 
of  the  naturalists  of  antiquity,  in  order  to  explain,  for  exam-' 
pie,  the  properties  of  the  lever,  took  refuge  in  the  assertion 
that  a  circle  is  such  a  marvellous  thing  that  no  wonder  if  mo- 
tions, taking  place  in  a  circle,  offer  also  in  their  turn  most 
unusual  phenomena.  If  Aristotle,  instead  of  straining  his 
extraordinary  powers  in  meditations  upon  the  fixed  point  and 
advancing  line,  as  he  calls  the  circle,  had  investigated  the 
numerical  relations  subsisting  between  the  length  of  the  arm 
of  the  lever  and  the  pressure  exerted,  he  would  have  laid  the 
foundation  of  an  important  part  of  human  knowledge. 

Such  mistakes,  committed  as  they  were,  in  accordance 
with  the  spirit  of  those  times,  even  by  a  man  whose  many 
positive  services  constitute  his  everlasting  memorial,  may 
serve  to  point  us  in  the  opposite  road  which  leads  us  surely 
to  the  goal.  But  if,  even  by  the  most  correct  method  of  in- 
vestigation, nothing  can  be  attained  without  toil  and  industry, 
the  cause  is  to  be  sought  in  that  divine  order  of  the  world 
according  to  which  man  is  made  to  labour.  But  it  is  certain 
that  already  immeasurably  more  means  and  more  toil  have 
been  sacrificed  to  error  than  were  needed  for  the  discovery  of 
the  truth. 


318     THE  MECHANICAL  EQUIVALENT  OF  HEAT. 

The  rule  which  must  be  followed,  in  order  to  lay  the  foun- 
dations of  a  knowledge  of  nature  in  the  shortest  conceivable 
time,  may  be  comprised  in  a  few  words.  The  natural  phe- 
nomena with  which  we  come  into  most  immediate  contact, 
and  which  are  of  most  frequent  occurrence,  must  be  subjected 
to  a  careful  examination  by  means  of  the  organs  of  sense, 
and  this  examination  must  be  continued  until  it  results  in 
quantitative  determinations  which  admit  of  being  expressed 
by  numbers. 

These  numbers  are  the  required  foundations  of  an  exact 
investigation  of  nature. 

Among  all  natural  operations,  the  free  fall  of  a  weight  is 
the  most  frequent,  the  simplest,  and — witness  Newton's  apple 
— at  the  same  time  the  most  important.  When  this  process 
is  analysed  in  the  way  that  has  been  mentioned,  we  imme- 
diately see  that  the  weight  strikes  against  the  ground  the 
harder  the  greater  the  height  from  which  it  has  fallen ;  and 
the  problem  now  consists  in  the  determination  of  the  quanti- 
tative relations  subsisting  between  the  height  from  which  the 
weight  falls,  the  time  occupied  by  it  in  its  descent,  and  its 
final  velocity,  and  in  expressing  these  relations  by  definite 
numbers. 

In  carrying  out  this  experimental  investigation,  various 
difficulties  have  to  be  contended  with ;  but  these  must  and 
can  be  overcome ;  and  then  the  truth  is  arrived  at,  that  for 
every  body  a  fall  of  sixteen  feet,  or  a  time  of  descent  of  one 
second,  corresponds  to  a  final  velocity  of  thirty-two  feet  per 
second. 

A  second  phenomenon  of  daily  occurrence,  which  is  in 
apparent  contradiction  to  the  laws  of  falling  bodies,  is  the 
ascent  of  liquids  in  tubes  by  suction.  Here,  again,  the  rule 
applies,  not  to  allow  the  maxim,  velle  rerum  cognoscere  causas, 
to  lead  us  into  error  through  useless  and  therefore  harmful 
speculations  concerning  the  qualities  of  the  vacuum,  and  the 
like  ;  on  the  contrary,  we  must  again  examine  the  phenome- 


THE   PROBLEM   OF   FALLING   BODIES.  319 

non  with  attention  and  awakened  senses ;  and  then  we  find, 
as  soon  as  we  put  a  tube  to  the  mouth  to  raise  a  liquid,  that 
the  operation  is  at  first  quite  easy,  but  that  afterwards  it  re- 
quires an  amount  of  exertion  which  rapidly  increases  as  the 
column  of  liquid  becomes  higher.  Is  there,  perchance,  an 
ascertainable  limit  to  the  action  of  suction  ?  As  soon  as  we 
once  begin  to  experiment  in  this  direction,  it  can  no  longer 
escape  us  that  there  is  a  barometric  height,  and  that  it  attains 
to  about  thirty  inches.  This  number  is  a  second  chief  pillar 
in  the  edifice  of  human  knowledge. 

Question  now  follows  question,  and  answer,  answer.  We 
have  learned  that  the  pressure  exerted  by  a  column  of  fluid  is 
proportional  to  its  height  and  to  the  specific  gravity  of  the 
fluid ;  we  have  thus  determined  the  specific  gravity  of  the  at- 
mosphere, and  by  this  investigation  we  are  led  to  carry  up 
our  measuring-instrument,  the  barometer,  from  the  plain  to 
the  mountains,  and  to  express  numerically  the  effect  produced 
by  elevation  above  the  sea-level  upon  the  height  of  the  mer- 
cury-column. Such  experiments  suggest  the  question,  Whether 
the  laws  of  falling  bodies,  with  which  we  have  become 
acquainted  at  the  surface  of  the  earth,  do  not  likewise  un- 
dergo modification  at  greater  distances  from  the  ground. 
And  if,  as  d  priori  we  cannot  but  expect,  this  should  be  really 
the  case,  the  further  question  arises,  In  what  manner  is  the 
number  already  found  modified  by  distance  from  the  earth  ? 
We  have  thus  come  upon  a  problem  the  solution  of  which  is 
attended  with  many  difficulties  ;  for  what  has  now  to  be  ac- 
complished, is  to  make  observations  and  carry  out  measure- 
ments in  places  where  no  human  foot  can  tread.  History, 
however,  teaches  that  the  same  man  who  put  the  question 
was  also  able  to  furnish  the  answer.  Truly  he  could  do  so 
only  through  a  rich  treasure  of  astronomical  knowledge.  But 
how  is  this  knowledge  to  be  attained  by  us  ? 

Astronomy  is,  without  question,  even  in  its  first  principles, 
the  most  difficult  of  all  sciences.  We  have  here  to  deal  with. 


THE  MECHANICAL   EQUIVALENT   OF   HEAT. 

objects  and  spaces  which  forbid  all  thought  of  experiment, 
while  at  the  same  time  the  motions  of  the  innumerable  heavr 
enly  bodies  are  of  so  complicated  a  kind,  that  astronomical 
science,  in  its  stately  unfolding,  is  rightly  considered  the  high- 
est triumph  whereof  human  intellect  here  below  is  able  to 
boast. 

In  accordance  with  the  natural  rule  that,  both  in  particu- 
lars and  in  general,  man  has  to  begin  with  that  which  is 
easiest  and  then  to  advance  step  by  step  to  what  is  more  diffi- 
cult, it  might  well  be  supposed  that  astronomy  must  have  ar- 
rived at  a  flourishing  state  of  development  later  than  any 
other  branch  of  human  knowledge.  But  it  is  well  known  that 
in  reality  the  direct  opposite  was  the  case,  inasmuch  as  it  was 
precisely  in  astronomy,  and  in  no  other  branch,  that  the  ear- 
liest peoples  attained  to  really  sound  knowledge,  It  may, 
indeed,  be  asserted  that  the  science  of  the  heavenly  bodies 
had  in  antiquity  reached  as  high  a  degree  of  perfection  as  the 
complete  want  of  all  the  auxiliary  sciences  rendered  possible. 

This  early  occurrence  of  a  vigorous  development  of  as- 
tronomy, which,  indeed,  was  a  necessary  forerunner  of  the 
other  sciences,  since  it  alone  furnished  the  necessary  data  for 
the  measurement  of  time,  is  observable  among  the  most  va- 
rious races  of  mankind :  the  reason  of  it,  moreover,  lies  in 
the  nature  of  things,  and  in  the  constitution  of  the  human 
mind.  It  furnishes  a  remarkable  proof  that  a  right  method 
is  the  most  important  condition  for  the  successful  prosecution 
of  scientific  inquiry. 

The  explanation  of  this  phenomenon  lies  in  the  fact  that 
the  need  which  was  felt  at  a  very  early  period,  of  a  common 
standard  for  the  computation  of  time,  made  it  necessary  to 
institute  observations  such  that  their  results  required  to  be 
expressed  by  definite  numbers.  There  was  a  felt  necessity  of 
determining  the  time  in  which  the  sun  accomplishes  his  cir- 
cuit through  the  heavens,  as  well  as  the  time  in  which  the 
moon  goes  through  her  phases,  and  other  similar  questions. 


FALLING   BODIES   AT   GREAT   HEIGHTS.  321 

In  order  to  meet  this  necessity,  there  was  no  temptation  to 
take  up  the  Book  of  Nature,  after  the  manner  of  expositors 
and  critics,  merely  to  cover  it  with  glosses  : 

"  Mit  eitler  Rede  wird  hier  nichts  geschaflft." 

It  was  numbers  that  were  sought,  and  numbers  that  were 
found.  The  overpowering  force  of  circumstances  constrained 
the  spirit  of  inquiry  into  the  right  path,  and  therein  led  it  at 
once  from  success  to  success. 

Now  that  after  long-continued,  accurate,  and  fortunate 
observations  the  needful  knowledge  of  the  courses  and  dis- 
tances of  the  nearest  heavenly  bodies,  as  well  as  of  the  figure 
and  size  of  the  earth,  has  been  acquired,  we  are  in  a  position 
to  treat  the  question,  What  is  the  numerical  influence  exerted 
by  increased  distance  from  the  earth  upon  the  known  laws  of 
falling  bodies  ?  and  we  thus  arrive  at  the  pregnant  discovery 
that,  at  a  height  equal  to  the  earth's  semidiameter,  the  dis- 
tance fallen  through  and  the  final  velocity,  for  the  first  second, 
is  four  times  less  than  on  the  surface  of  the  earth. 

In  order  to  pursue  our  inquiry,  let  us  now  return  to  the 
objects  which  immediately  surround  us.  From  the  earliest 
times,  the  phenomena  of  combustion  must  have  claimed  in  an 
especial  degree  the  attention  of  mankind.  In  order  to  ex- 
plain them,  the  ancients,  in  accordance  with  the  method  of 
their  naturalistic  philosophy,  put  forward  a  peculiar  upward- 
striving  element  of  Fire,  which  in  conjunction  with,  and  in 
opposition  to,  Air,  Water,  and  Earth,  constituted  all  that  ex- 
isted. The  necessary  consequence  of  this  theory,  which  they 
discussed  with  the  most  acute  sagacity,  was,  that  in  regard  to 
the  phenomena  in  question  and  all  that  related  to  them,  they 
remained  in  complete  ignorance. 

Here,  again,  it  is  quantitative  determinations,  it  is  num- 
bers alone,  which  put  the  Ariadne's  clue  in  our  hand.     If  we 
want  to  know  what  goes  on  during  the  phenomena  of  com- 
bustion, we  must  weigh  the  substances  before  and  after  they 
14* 


THE  MECHANICAL   EQUIVALENT  OF   HEAT. 

are  burned ;  and  here  the  knowledge  we  have  already  ac- 
quired of  the  weight  of  gaseous  bodies  comes  to  our  aid. 
"We  then  find  that,  in  every  case  of  combustion,  substances 
which  previously  existed  in  a  separate  state  enter  into  an  inti- 
mate union  with  each  other,  and  that  the  total  weight  of  the 
substances  remains  the  same  both  before  and  after  the  combi- 
nation. "We  thus  come  to  know  the  different  bodies  in  their 
separate  and  in  their  combined  states,  and  learn  how  to  trans- 
form them  from  one  of  these  states  into  the  other ;  we  learn, 
for  instance,  that  water  is  composed  of  two  kinds  of  air  which 
combine  with  each  other  in  the  proportion  of  1:  8.  An  en- 
trance into  chemical  science  is  thus  opened  to  us,  and  the  nu- 
merical laws  which  regulate  the  combinations  of  matter  (die 
Stochiometrie)  hang  like  ripe  fruit  before  us. 

As  we  proceed  further  in  our  investigations,  we  find  that 
in  all  chemical  operations — combinations  as  well  as  decompo- 
sitions— changes  of  temperature  occur,  which,  according  to 
the  varying  circumstances  of  different  cases,  are  of  all  de- 
grees of  intensity,  from  the  most  violent  heat  downwards. 
We  have  measured  quantitatively  the  heat  developed,  or 
counted  the  number  of  heat-units,  and  have  so  come  into  pos- 
session of  the  law  of  the  evolution  of  heat  in  chemical  pro- 
cesses. 

We  have  long  known,  however,  that  in  innumerable  cases 
heat  makes  its  appearance  where  no  chemical  action  is  going 
on ;  for  instance,  whenever  there  is  friction,  when  unelastic 
bodies  strike  one  another,  and  when  aeriform  bodies  are  com- 
pressed. 

What  then  takes  place  when  heat  is  evolved  in  such  ways 
as  these  f 

We  are  taught  by  history  that  in  this  case  also  the  most 
sagacious  hypotheses  concerning  the  state  and  nature  of  a 
peculiar  "matter"  of  heat,  concerning  a  "thermal  aether," 
whether  at  rest  or  in  a  state  of  vibration,  concerning  "  ther- 
mal atoms,"  supposed  to  exercise  their  functions  in  the  inter- 


CONVERTIBILITY   OF   HEAT  AND  MOTION.  323 

slices  between  the  material  atoms,  or  other  hypotheses  of  like 
nature,  have  not  availed  to  solve  the  problem.  It  is,  notwith- 
standing, of  no  less  wonderfully  simple  a  nature  than  the  laws 
of  the  lever,  about  which  the  founder  of  the  peripatetic  phi- 
losophy cudgelled  his  brains  in  vain. 

After  what  has  gone  before,  the  reader  cannot  be  in  any 
doubt  about  what  is  the  course  now  to  be  pursued.  We  must 
again  make  quantitative  determinations:  we  must  measure 
and  count. 

If  we  proceed  in  this  direction  and  measure  the  quantity 
of  heat  developed  by  mechanical  agency,  as  well  as  the 
amount  of  force  used  up  in  producing  it,  and  compare  these 
quantities  with  each  other,  we  at  once  find  that  they  stand  to 
each  other  in  the  simplest  conceivable  relation — that  is  to  say, 
in  an  invariable  direct  proportion,  and  that  the  proportion  also 
holds  when,  inversely,  mechanical  force  is  again  produced  by 
the  aid  of  heat. 

Putting  these  facts  into  brief  and  plain  language,  we  may 
say, 

Heat  and  motion  are  transformable  one  into  the  other. 

We  cannot  and  ought  not,  however,  to  let  this  suffice  us. 
We  require  to  know  how  much  mechanical  force  is  needed  for 
the  production  of  a  given  amount  of  heat,  and  conversely. 
In  other  words,  the  law  of  the  invariable  quantitative  relation 
between  motion  and  heat  must  be  expressed  numerically. 

When  we  appeal  hereupon  to  experiment,  we  find  that 
raising  the  temperature  of  a  given  weight  of  water  one  degree 
of  the  Centigrade  scale  corresponds  to  the  elevation  of  an 
equal  weight  to  the  height  of  about  1,200  [French]  feet. 

This  number  is  THE  MECHANICAL  EQUIVALENT  OF  HEAT. 

The  production  of  heat  by  friction  and  other  mechanical 
operations  is  a  fundamental  fact  of  such  constant  occurrence, 
that  the  importance  of  its  establishment  on  a  scientific  basis 
will  be  recognized  by  naturalists  without  any  preliminary 


324:     THE  MECHANICAL  EQUIVALENT  OF  HEAT. 

enumeration  of  its  useful  applications ;  and,  for  the  same 
reason,  a  few  historical  remarks  touching  the  circumstances 
attending  the  discovery  of  the  foregoing  fundamental  law, 
will  not  be  out  of  place  here. 

In  the  summer  of  1840,  on  the  occasion  of  bleeding  Eu- 
ropeans newly  arrived  in  Java,  I  made  the  observation  that 
the  blood  drawn  from  the  vein  of  the  arm  possessed,  almost 
without  exception,  a  surprisingly  bright  red  colour. 

This  phenomenon  riveted  my  earnest  attention.  Starting 
from  Lavoisier's  theory,  according  to  which  animal  heat  is 
the  result  of  a  process  of  combustion,  I  regarded  the  twofold 
change  of  colour  which  the  blood  undergoes  in  the  capillaries 
as  a  sensible  sign — as  the  visible  indication — of  an  oxidation 
going  on  in  the  blood.  In  order  that  the  human  body  may 
be  kept  at  a  uniform  temperature,  the  development  of  heat 
within  it  must  bear  a  quantitative  relation  to  the  heat  which 
it  loses — a  relation,  that  is,  to  the  temperature  of  the  sur- 
rounding medium ;  and  hence  both  the  production  of  heat 
and  the  process  of  oxidation,  as  well  as  the  difference  in  col- 
our of  tlie  two  kinds  of  blood,  must  be  on  the  whole  less  in 
the  torrid  zones  than  in  colder  regions. 

In  accordance  with  this  theory,  and  having  regard  to  the 
known  physiological  facts  which  bear  upon  the  question,  the 
blood  must  be  regarded  as  a  fermenting  liquid  undergoing 
slow  combustion,  whose  most  important  function — that  is, 
sustaining  the  process  of  combustion — is  fulfilled  without  the 
constituents  of  the  blood  (with  the  exception,  that  is,  of  the 
products  of  decomposition)  leaving  the  cavities  of  the  blood- 
vessels or  coming  into  such  relation  with  the  organs  that  an 
interchange  of  matter  can-  take  place.  This  may  be  thus 
stated  in  other  words  :  by  far  the  greater  part  of  the  assimi- 
lated food  is  burned  in  the  cavities  of  the  blood-vessels  them- 
selves, for  the  purpose  of  producing  a  physical  effect,  and  a 
comparatively  small  quantity  only  serves  the  less  important 
end  of  ultimately  entering  the  substance  of  the  organs  them- 


PROBLEM   OF   PHYSIOLOGICAL   HEAT.  325 

selves,  so  as  to  occasion  growth  and  the  renewal  of  the  worn- 
out  solid  parts. 

If  hence  it  follows  that  a  general  balance  must  be  struck 
in  the  organism  between  receipts  and  expenditure,  or  between 
work  done  and  wear  and  tear,  it  is  unmistakably  one  of  the 
most  important  problems  with  which  the  physiologist  has  to 
deal,  to  make  himself  as  thoroughly  acquainted  as  it  is  possi- 
ble for  him  to  be  with  the  budget  of  the  object  of  his  exami- 
nation. The  wear  and  tear  consists  in  the  amount  of  matter 
consumed ;  the  work  done  is  the  evolution  of  heat.  This 
latter  effect,  however,  is  of  two  kinds,  inasmuch  as  the  ani- 
mal body  evolves  heat  on  the  one  hand  directly  in  its  own 
interior,  and  distributes  it  by  communication  to  the  objects 
immediately  surrounding  it;  while,  on  the  other  hand,  it 
possesses,  through  its  organs  of  motion,  the  power  of  produc- 
ing heat  mechanically  by  friction  or  in  similar  ways,  even  at 
distant  points.  We  now  require  to  know 

Whether  the  heat  directly  evolved  is  ALONE  to  he  laid  to  the 
account  of  the  process  of  combustion,  or  whether  it  is  the  SUM 
of  the  heat  evolved  loth  directly  and  indirectly  that  it  is  to  be 
taken  into  calculation. 

This  is  a  question  that  touches  the  very  foundations  of  sci- 
ence ;  and  unless  it  receives  a  trustworthy  answer,  the  healthy 
development  of  the  doctrine  concerned  is  not  possible.  For 
it  has  been  already  shown,  by  various  examples,  what  are 
the  consequences  of  neglecting  primary  quantitative  determi- 
nations. No  wit  of  man  is  able  to  furnish  a  substitute  for 
what  nature  offers. 

The  physiological  theory  of  combustion  starts  from  the 
fundamental  proposition,  that  the  quantity  of  heat  which  re- 
sults from  the  combustion  of  a  given  substance  is  invariable — 
that  is,  that  its  amount  is  uninfluenced  by  the  circumstances 
which  accompany  the  combustion  ;  whence  we  infer,  "  in  spe- 
cie" that  the  chemical  effect  of  combustible  matter  can  un- 
dergo no  alteration  in  amount  even  by  the  vital  process,  or 


326  THE   MECHANICAL   EQUIVALENT  OF   HEAT. 

that  the  living  organism,  with  all  its  riddles  and  marvels,  can- 
not create  heat  out  of  nothing. 

But  if  we  hold  firm  to  this  physiological  axiom,  the  an- 
swer to  the  question  started  above  is  already  given.  For, 
unless  we  wish  to  attribute  again  to  the  organism  the  power 
of  creating  heat  which  has  just  been  denied  to  it,  it  cannot  be 
assumed  that  the  heat  which  it  produces  can  ever  amount  to 
more  than  the  chemical  action  which  takes  place.  On  the 
combustion-theory  there  is,  then,  no  alternative,  short  of  sa- 
crificing the  theory  itself,  but  to  admit  that  the  total  amount 
of  heat  evolved  by  the  organism,  partly  directly,  and  partly 
indirectly  by  mechanical  action,  corresponds  quantitatively, 
or  is  equal  to  the  amount  of  combustion. 

Hence  it  follows,  no  less  inevitably,  that  the  heat  produced 
mechanically  by  the  organism  must  bear  an  invariable  quantita- 
tive relation  to  the  work  expended  in  producing  it. 

For  if,  according  to  the  varying  construction  of  the  me- 
chanical arrangements  which  serve  for  the  development  of 
the  heat,  the  same  amount  of  work,  and  hence  the  same 
amount  of  organic  combustion,  could  produce  varying  quanti- 
ties of  heat,  the  quantity  of  heat  produced  from  one  and  the 
same  expenditure  of  material  would  come  out  smaller  at  one 
time  and  larger  at  another,  which  is  contrary  to  our  assump- 
tion. Further,  inasmuch  as  there  is  no  difference  in  kind  be- 
tween the  mechanical  performances  of  the  animal  body  and 
those  of  other  inorganic  sources  of  work,  it  follows  that 

AN  INVARIABLE  QUANTITATIVE  RELATION  BETWEEN  HEAT 
AND  WOKK  IS  A  POSTULATE  OF  THE  PHYSIOLOGICAL  THEORY 
OF  COMBUSTION. 

While  following  in  general  the  direction  indicated,  it  was 
accordingly  needful  for  me  in  the  end  to  fix  my  attention 
chiefly  on  the  physical  connection  subsisting  between  motion 
and  heat ;  and  it  was  thus  impossible  for  the  existence  of  the 
mechanical  equivalent  of  heat  to  remain  hidden  from  me. 
But,  although  I  have  to  thank  an  accident  for  this  discovery, 


BELATION  BETWEEN   HEAT  AND   WOKK.  327 

it  is  none  the  less  my  own,  and  I  do  not  hesitate  to  assert  my 
right  of  priority. 

In  order  to  ensure  what  had  been  thus  discovered  against 
casualties,  I  put  together  the  most  important  points  in  a  short 
paper  which  I  sent  in  the  spring  of  1842  to  Liebig,  with  a 
request  that  he  would  insert  it  in  the  Annalen  der  Chemie  und 
Pharmacie,  in  the  forty-second  volume  of  which,  page  233,  it 
may  be  found  under  the  title  "  Bemerkungen  iiber  die  Krafte 
der  unbelebten  Natur." 

It  was  a  fortunate  circumstance  for  me  that  the  reception 
given  to  my  unpretending  work  by  this  man,  gifted  with  so 
deep  an  insight,  at  once  secured  for  it  an  entrance  into  one  of 
the  first  scientific  organs,  and  I  seize  this  opportunity  of  pub- 
licly testifying  to  the  great  naturalist  my  gratitude  and  my 
esteem. 

Liebig  himself,  however,  had  about  the  same  time  already 
pointed  out,  in  more  general  but  still  unmistakable  terms,  the 
connection  subsisting  between  heat  and  work.  In  particular, 
he  asserts  that  the  heat  produced  mechanically  by  a  steam- 
engine  is  to  be  attributed  solely  to  the  effect  of  combustion, 
which  can  never  receive  any  increase  through  the  fact  of  its 
producing  mechanical  effects,  and,  through  these,  again  devel- 
oping heat. 

From  these,  and  from  similar  expressions  of  other  scien- 
tific men,  we  may  infer  that  science  has  recently  entered  upon 
a  direction  in  which  the  existence  of  the  mechanical  equiva- 
lent of  heat  could  not  in  any  case  have  remained  longer  un- 
perceived. 

In  the  paper  to  which  reference  has  been  made,  the  nat- 
"ural  law  with  which  we  are  now  concerned  is  referred  back 
to  a  few  fundamental  conceptions  of  the  human  mind.  The 
proposition  that  a  magnitude,  which  does  not  spring  from 
nothing,  cannot  be  annihilated,  is  so  simple  and  clear  that  no 
valid  argument  can  be  urged  against  its  truth,  any  more  than 
against  an  axiom  of  geometry ;  and  until  the  contrary  is 


328     THE  MECHANICAL  EQUIVALENT  OF  HEAT. 

proved  by  some  fact  established  beyond  a  doubt,  we  may  ac- 
cept it  as  true. 

Now  we  are  taught  by  experience,  that  neither  motion  nor 
heat  ever  takes  its  rise  except  at  the  expense  of  some  meas- 
urable object,  and  that  in  innumerable  cases  motion  disap- 
pears without  any  thing  except  heat  making  its  appearance. 
The  axiom  that  we  have  established  leads,  then,  now  to  the 
conclusion  that  the  motion  that  disappears  becomes  heat,  or, 
in  other  words,  that  both  objects  bear  to  each  other  an  inva- 
riable quantitative  relation.  The  proof  of  this  conclusion  by 
the  method  of  experiment,  the  establishment  of  it  in  all  its 
details,  the  tracing  of  a  complete  harmony  subsisting  between 
the  laws  of  thought  and  the  objective  world,  is  the  most  inter- 
esting, but  at  the  same  time  the  most  comprehensive  problem 
that  it  is  possible  to  find.  What  I,  with  feeble  powers  and 
without  any  external  support  or  encouragement,  have  effected 
in  this  direction  is  truly  little  enough  ;  but — ultra  posse  nemo 
olligatus. 

In  the  paper  referred  to  (the  first  of  Mayer's  in  the  pres- 
ent volume)  I  have  thus  expressed  myself  with  regard  to  the 
genetic  connection  of  heat  and  moving  force  : 

"If  it  be  now  considered  as  established  that  in  many 
cases  (exceptio  confirmat  regulam)  no  other  effect  of  motion 
can  be  traced  except  heat,  and  that  no  other  cause  than  motion 
can  be  found  for  the  heat  that  is  produced,  we  prefer  the  as- 
sumption that  heat  proceeds  from  motion,  to  the  assumption 
of  a  cause  without  effect  and  of  an  effect  without  a  cause — 
just  as  the  chemist,  instead  of  allowing  oxygen  and  hydrogen 
to  disappear  without  further  investigation,  and  water  to  be 
produced  in  some  inexplicable  manner,  establishes  a  connec- 
tion between  oxygen  and  hydrogen  on  the  one  hand  and  water 
on  the  other." 

From  this  point  there  is  but  one  step  to  be  made  to  the 
goal.  At  page  257  it  is  said :  "  The  solution  of  the  equa- 
tions subsisting  between  falling-force  [that  is,  the  raising  of 


CONNECTION   OF   HEAT  AND  MOVING  FORCE.          329 

weight]  and  motion  requires  that  the  space  fallen  through  in 
a  2;iven  time,  e.  g.  the  first  second,  should  be  experimentally 
determined ;  in  like  manner,  the  solution  of  the  equations 
subsisting  between  falling-force  and  motion  on  the  one  hand 
and  heat  on  the  other,  requires  an  answer  to  the  question,  How 
great  is  the  quantity  of  heat  which  corresponds  to  a  given 
quantity  of  motion  or  falling-force  ?  For  instance,  we  must 
ascertain  how  high  a  given  weight  requires  to  be  raised  above 
the  ground  in  order  that  its  falling-force  may  be  equivalent  to 
the  raising  of  the  temperature  of  an  equal  weight  of  water 
from  0°  to  1°  C.  The  attempt  to  show  that  such  an  equation 
is  the  expression  of  a  physical  truth  may  be  regarded  as  the 
substance  of  the  foregoing  remarks. 

"  By  applying  the  principles  that  have  been  set  forth  to 
the  relations  subsisting  between  the  temperature  and  the  vol- 
ume of  gases,  we  find  that  the  sinking  of  a  mercury  column 
by  which  a  gas  is  compressed  is  equivalent  to  the  quantity  of 
heat  set  fire  by  the  compression ;  and  hence  it  follows,  the 
ratio  between  the  capacity  for  heat  of  air  under  constant  press- 
ure and  its  capacity  under  constant  volume  being  taken  as 
=  1-421,  that  the  warming  of  a  given  weight  of  water  from 
0°  to  1°  C.  corresponds  to  the  fall  of  an  equal  weight  from 
the  height  of  about  365  metres." 

It  is  plain  that  the  expression  "  equivalent "  is  here  used 
in  quite  a  different  sense  from  what  it  bears  in  chemistry. 
The  difference  will  be  shown  most  distinctly  by  an  example. 
When  the  same  weight  of  potash  is  neutralized,  first,  with 
sulphuric  acid,  then  with  nitric  acid,  the  numbers  which  ex- 
press the  ratio  which  the  absolute  weights  of  these  three  sab- 
stances  bear  to  one  another  are  called  their  equivalents ;  but 
there  is  no  thought  here  either  of  the  quantitative  equality  or 
of  the  transformation  of  the  bodies  in  question. 

This  peculiar  signification  which  the  word  "  equivalent" 
has  acquired  in  chemistry,  is  doubtless  connected  with  the 
fact  that  the  chemist  has  been  able  to  determine  the  object  of 


330  THE  MECHANICAL   EQUIVALENT  OF   HEAT. 

his  investigation  by  a  common  quantitative  standard,  their  ab- 
solute weights.  Let  us  suppose,  however,  that  we  could  de- 
termine one  body,  for  instance  water,"  only  by  weight,  and 
another,  water-forming  or  explosive  gas,  only  by  volume,  and 
that  we  had  agreed  to  choose  one  pound  as  the  unit  of  weight, 
and  one  cubic  foot  as  the  unit  of  volume  ;  we  should  then  have 
to  ascertain  how  many  cubic  feet  of  explosive  gas  could  be  ob- 
tained from  one  pound  of  water,  and  conversely.  This  num- 
ber, without  which  neither  the  formation  nor  the  decomposi- 
tion of  water  could  be  made  the  subject  of  calculation,  might 
then  be  suitably  called  "  the  explosive-gas  equivalent  of  wa- 
ter." 

In  this  latter  sense  a  raised  weight  might,  in  accordance 
with  the  known  laws  of  mechanics,  be  called  the  "  equiva- 
lent "  of  the  motion  resulting  from  its  fall.  Now,  in  order  to 
compare  these  two  objects,  the  raised  and  the  moving  weight, 
which  admit  of  no  common  measure,  we  require  that. con- 
stant number  which  is  generally  denoted  by  g.  This  number, 
however,  and  the  mechanical  equivalent  of  heat,  whereby  the 
relation  subsisting  between  heat  and  motion  is  defined,  belong 
both  of  them  to  one  and  the  same  category  of  ideas. 

In  the  paper  that  I  have  mentioned  it  is  further  shown 
how  we  may  arrive  at  such  a  conception  of  force  as  admits 
of  being  consistently  followed  to  its  consequences  and  is  sci- 
entifically tenable  ;  and  the  importance  of  this  subject  induces 
me  to  return  to  it  again  here. 

The  word  "  force"  (Kraft)  is  used  in  the  higher  or  scien- 
tific mechanics  in  two  distinct  senses. 

I.  On  the  one  hand,  it  denotes  every  push  or  pull,  every 
effort  of  an  inert  body  to  change  its  state  of  rest  or  of  mo- 
tion ;  and  this  effort,  when  it  is  considered  alone  and  apart 
from  the  result  produced,  is  called  "  pushing  force,"  "  pulling 
force,"  or  shortly  "  force,"  and  also,  in  order  to  distinguish 
between  this  and  the  following  conception,  "  dead  force"  (vis 
mortua) . 


MEANING  OF  THE  TEKM   "FOBCE."  331 

IE.  On  the  other  hand,  the  product  of  the  pressure  into 
the  space  through  which  it  acts,  or,  again,  the  product — or 
half-product — of  the  mass  into  the  square  of  the  velocity,  is 
named  "  force."  In  order  that  motion  may  actually  occur,  it 
is  in  fact  necessary  that  the  mass,  whatever  it  may  be,  should 
under  the  influence  of  a  pressure,  and,  in  the  direction  of  that 
pressure,  traverse  a  certain  space,  "the  effective  space" 
(  WirJcungsraum)  :  and  in  this  case  a  magnitude  which  is  pro- 
portional to  the  "  pushing  force "  and  to  the  effective  space, 
likewise  receives  the  name  "force;"  but  to  distinguish  it 
from  the  mere  pushing  force,  by  which  alone  motion  is  never 
actually  brought  about,  it  is  also  called,  the  "  vis  viva  of  mo- 
tion," or  "  moving  force." 

With  the  generic  conception  of  "  force,"  the  higher  me- 
chanics, as  an  essentially  analytic  science,  is  not  concerned. 
In  order  to  arrive  at  it,  we  must,  according  to  the  general 
rule,  collect  together  the  characters  possessed  in  common  by 
the  several  species.  As  is  well  known,  the  definition  so  ob- 
tained runs  thus — "  Force  is  every  thing  which  brings  about 
or  tends  to  bring  about,  alters  or  tends  to  alter  motion." 

This  definition,  however,  it  is  easy  to  see,  is  tautological ; 
for  the  last  fourteen  words  of  it  might  be  omitted,  and  the 
sense  would  be  still  the  same. 

This  erroneous  solution  is  occasioned  by  the  nature  of  the 
problem,  which  requires  an  impossibility.  Mere  pressure 
(dead  force)  and  the  product  of  the  pressure  into  the  effective 
space  (living  force)  are  magnitudes  too  thoroughly  unlike  to 
be  by  possibility  combined  into  a  generic  conception.  Press- 
ure or  attraction  is,  in  the  theory  of  motion,  what  affinity  is 
in  chemistry — an  abstract  conception :  living  force,  like  mat- 
ter, is  concrete  ;  and  these  two  kinds  of  force,  however  closely 
connected  in  the  region  of  the  association  of  ideas,  are  in 
reality  so  widely  separated  that  a  frame  which  should  take 
them  both  in  must  be  able  to  include  the  whole  world. 

There  are  several  conceivable  ways  of  escaping  from  the 


332  THE  MECHANICAL   EQUIVALENT  OF   HEAT. 

difficulty.  For  instance,  just  as  we  speak  of  absolute  weight, 
specific  weight,  and  combining  weight,  without  its  ever  enter- 
ing any  one's  head  to  want  to  construct  a  generic  idea  out  of 
these  distinct  notions,  so  two  or  more  meanings  may  be  at- 
tached to  the  word  force.  This  is  what  is  actually  done  in 
the  higher  mechanics,  and  hence  in  this  branch  of  science  we 
meet  with  no  mention  of  a  generic  conception  of  "  force." 

There  has  been  no  lack  of  recommendations  to  carry,  in 
like  manner,  the  notions  of  "  dead"  and  "  living  force"  as 
distinct  and  separate  through  the  other  departments  of  sci- 
ence ;  it  has,  however,  been  found  impossible  to  put  in  prac- 
tice .  such  recommendations ;  for  the  use  of  ambiguous  ex- 
pressions, which  can  in  no  case  contribute  any  thing  to  clear- 
ness, is  altogether  inadmissible  if  confusion  can  possibly  arise. 
It  is  true  that  the  mathematician  is  in  no  danger  of  confound- 
ing in  his  calculations  a  product  with  one  of  its  factors  ;  but 
in  other  departments  of  knowledge  a  systematic  confusion  of 
ideas  exists  on  this  point ;  and  if  any  thing  is  to  be  done 
toward  clearing  it  up>  the  source  of  the  error  must  be  stopped  ; 
for  if  we  once  recognize  two  meanings  of  the  word  "  force," 
it  would  be  the  labour  of  Sisyphus  to  try  to  distinguish  be- 
tween them  in  each  separate  case.  In  order,  then,  to  arrive 
at  any  result,  we  must  make  up  our  minds  to  do  without  any 
common  denomination  of  the  magnitudes  mentioned  above, 
as  I.  and  II.,  and  either  to  give  up  the  use  of  the  word 
"  force"  altogether,  or  to  employ  it  for  one  only  of  these  two 
categories. 

The  notion  of  force  was  consistently  employed  in  the  lat- 
ter sense  by  Newton.  In  solving  his  problems,  he  decom- 
poses the  product  of  the  attraction  into  the  effective  space 
into  its  two  factors,  and  calls  the  former  by  the  name  "  force." 

As  an  objection  to  this  mode  of  proceeding,  it  must,  how- 
ever, be  remarked  that  in  many  cases  it  is  not  possible  thus 
to  decompose  the  product  in  question.  Let  us  take,  for  in- 
stance, the  following  very  simple  case  :  a  mass  M,  originally 


MEAIHNG   OF  THE  TEEM   "FOKCE."  333 

at  rest,  is  caused  to  move  with  the  (uniform  final)  velocity  c  ; 
from  the  knowledge  of  the  magnitudes  M  and  c  it  is  certainly 
possible  to  deduce  the  value  of  the  product  of  the  force  (in 
Newton's  sense)  into  its  effective  space,  but  we  are  not  thereby 
enabled  to  conclude  as  to  the  magnitude  of  this  force  itself. 

As  a  matter  of  fact,  the  necessity  soon  made  itself  felt  of 
treating  and  naming  this  product  as  a  whole.  It  also  has 
been  called  "  force,"  and  the  expressions  "  vis  viva  of  motion," 
" moving  force,"  "working  force,"  " horse-power  "  (or  force), 
"  muscular  force,"  &c.,  have  been  long  naturalized  in  science. 

However  happy  we  may,  in  many  respects,  think  the 
choice  of  this  word,  there  is  still  the  objection  that  a  Hew 
meaning  has  been  fixed  upon  an  already  existing  technical 
expression,  without  the  old  one  having  been  called  in  from 
circulation  at  the  same  time.  This  formal  error  has  become 
a.  Pandora's  box,  whence  has  sprung  a  Babylonian  confusion 
of  tongues. 

Under  existing  circumstances  no  choice  is  left  us  but  to 
withdraw  the  term  "  force"  either  from  Newton's  dead  force 
or  from  Leibnitz's  living  force  ;  but  in  either  case  we  come 
into  conflict  with  prevailing  usage.  But  if  once  we  have 
made  up  our  minds  to  introduce  into  our  science  a  logically 
accurate  use  of  terms,  even  at  the  cost  of  existing  expressions 
which  have  become  easy  and  pleasant  to  us  by  long  usage,  we 
cannot  long  hesitate  in  the  choice  we  have  to  make  between 
the  conceptions  I.  and  II. 

Let  us  consider  the  elementary  case  of  a  mass,  originally 
at  rest,  which  receives  motion :  this  happens,  as  has  been  al- 
ready said,  by  the  mass  being  subjected  to  a  certain  push  or 
pull  under  the  influence  of  which  it  traverses  a  certain  space, 
the  effective  space.  Now,  however,  both  the  velocity  and 
also  the  intensity  of  the  push  (Newton's  force)  always  vary 
at  every  point  of  the  effective  space ;  and  in  order  to  mul- 
tiply these  variable  magnitudes  into  effective  space,  that  is, 
to  deduce  the  quantity  of  motion  from  the  intensity  of  the 


334     THE  MECHANICAL  EQUIVALENT  OF  HEAT. 

pushing  force,  we  must  call  in  the  aid  of  the  higher  mathe- 
matics. 

But  hence  it  follows  that,  except  in  statics,  where  the  ef- 
fective space  is  nought  and  the  pressure  constant,  the  New- 
tonian conception  of  force  is  available  only  in  the  higher 
branches  of  mechanics  ;  and  it  is  plainly  not  advisable  so  to 
choose  our  conception  of  "  force  "  that  it  cannot  be  consist 
ently  employed  in  that  branch  (namely,  the  elementary  parts 
of  the  theory  of  motion)  which  of  all  others  is  chiefly  con- 
cerned with  fundamental  notions. 

It  is,  however,  a  totally  mistaken  method  to  try  to  adapt 
the -idea  of  a  force,  such  as  gravity,  conceived  in  Newton's 
sense,  to  the  elementary  parts  of  science,  by  leaving  out  of 
consideration  one  of  its  most  important  properties,  namely  its 
dependence  on  distance,  and  to  make  a  "  force  "  out  of  Gali- 
leo's gravity  thus  inexactly  and  in  some  relations  most  incor- 
rectly conceived.  Some  such  ideal  force  (No.  III.)  seems 
to  hover  before  the  minds  of  most  writers  on  natural  science 
as  the  original  type  of  a  "force  of  nature." 

Such  quantitative  determinations  as  hold  good  only  ap- 
proximately and  under  certain  conditions  ought  never  to  be 
employed  to  establish  definitions.  In  a  calculation,  it  is  true, 
we  may  correctly  enough  take  an  arc,  which  is  sufficiently 
small  in  comparison  with  the  radius,  as  equal  in  size  to  the 
sine  or  to  the  tangent ;  but  if  we  attempted  to  use  such  a  rela- 
tion in  settling  first  principles,  we  should  lay  a  foundation  for 
fallacies  and  errors. 

The  Newtonian  idea  of  force,  however,  transplanted  in 
the  manner  that  is  commonly  done  into  the  region  of  element- 
ary science,  is  no  whit  better  than  the  notion  of  a  straight 
curve.  Newton's  force,  or  attraction,  in  specie  gravity,  g,  is 
equal  to  the  differential  quotient  of  the  velocity  by  the  time  ; 

dc 

that  is,  0=-r'     This  expression  is  quite  exact,  but  in  order  to 
cit 

understand  and  apply  it  a  knowledge  of  the  higher  mathemat- 


NEWTON'S  CONCEPTION  OF  FORCE.  335 

ics  is  required.  On  the  other  hand,  it  is  quite  true  that,  so 
long  as  we  have  to  do  only  with  cases  in  which  the  space 
fallen  through  is  so  small  in  comparison  with  the  earth's  semi- 
diameter  that  it  may  be  disregarded,  the  equation  just  given 

c 

may  be  abbreviated  into  the  very  convenient  form  g=~i  with- 
out any  considerable  error ;  but  this  expression  can  never  be 
mathematically  exact  so  long  as  the  space  fallen  through  has 
any  calculable  magnitude.  But  on  the  strength  of  an  equa- 
tion thus  radically  inaccurate,  there  are  planted  in  the  recep- 
tive mind  of  youth  such  false  notions  as — that  gravity  is  a 
uniformly  accelerating  (  ?)  force,  a  moving  (  ?)  force  whose  ac- 
tion is  proportional  to  the  time  (  ?)  ;  that  force  is  directly  pro- 
portional (  ?)  to  the  velocity  produced ;  and  many  other  like 
errors. 

It  would  certainly  be  a  great  merit  if  authors  of  treatises 
on  physics  would  help  to  remedy  this  state  of  things,  and  in 
framing  their  definitions  would  start  only  from  thoroughly 
exact  determinations  of  magnitudes ;  for  elementary  physics 
in  its  present  form,  instead  of  being  a  well-grounded  science, 
is  only  a  sort  of  half-knowledge,  such  that  on  passing  to  the 
higher  and  strictly  scientific  departments  the  student  must 
try  to  forget  its  principles  and  theorems  as  quickly  as  he  can. 

If  we  have  once  convinced  ourselves  by  unprejudiced 
examination  that  the  retention,  under  that  name,  of  the  con- 
ception of  force  distinguished  above  by  I.  has  nothing  but  its 
origin  to  recommend  it,  but  much  to  condemn  it,  the  rest  fol- 
lows almost  spontaneously.  It  accords  with  the  laws  of 
thought,  as  well  as  with  the  common  usage  of  language,  to 
connect  every  production  of  motion  with  an  expenditure  of 
force.  Hence  "  force  "  is — 

Something  which  is  expended  in  producing  motion;  and 
this  something  which  is  expended  is  to  be  looked  upon  as  a 
cause  equivalent  to  the  effect,  namely,  to  the  motion  pro- 
duced. 


336     THE  MECHANICAL  EQUIVALENT  OF  HEAT. 

This  definition  not  only  corresponds  perfectly  with  facts, 
but  it  accords  as  far  as  possible  with  that  which  already  ex- 
ists ;  for,  as  I  shall  show,  it  contains  by  implication  the  con- 
ception of  force  as  met  with  in  the  higher  mechanics,  and  re- 
ferred to  above  by  II. 

If  a  mass  M,  originally  at  rest,  while  traversing  the  effect- 
ive space  s,  under  the  influence  and  in  the  direction  of  the 
pressure  j),  acquires  the  velocity  c,  we  have£>s=Mca.  Since, 
however,  every  production  of  motion  implies  the  existence  of 
a  pressure  (or  of  a  pull)  and  an  effective  space,  and  also  the 
exhaustion  of  one  at  least  of  these  factors,  the  effective  space, 
it  follows  that  motion  can  never  come  into  existence  except 
at  the  cost  of  this  product,  ps=M.c*.  And  this  it  is  which 
for  shortness  I  call  "  force." 

The  connection  between  expenditure  and  performance  (in 
other  words,  the  exhaustion  of  force  in  producing  its  effect) 
presents  itself  in  the  simplest  form  in  the  phenomena  of  grav- 
itation. The  necessary  condition  of  every  falling  motion  is 
that  the  centre  of  gravity  of  the  two  masses  concerned  in  it 
(that  is,  of  the  earth  and  of  the  falling  weight)  should  ap- 
proach each  other.  But  in  the  case  of  the  falling  together  of 
the  two  masses,  the  approach  of  their  centres  of  gravity 
reaches  its  natural  limit,  and  hence  the  production  of  a  fall- 
ing movement  is  thus  bound  up  with  an  expenditure,  namely, 
with  the  exhaustion  of  the  given  falling-space,  and  thereby 
also  of  the  product  of  that  space  into  the  attraction.  The 
falling  down  of  a  weight  upon  the  earth  is  a  process  of  me- 
chanical combination  ;  and  just  as  in  combustion  the  capacity 
of  performance  (that  is,  the  condition  of  the  development  of 
heat)  ceases  when  the  act  of  combination  comes  to  an  end,  so 
also  the  production  of  motion  ceases  when  the  weight  has 
fallen  to  its  lowest  position.  The  weight,  when  lying  on  the 
solid  ground,  is,  like  the  carbonic  acid  formed  in  combustion, 
nothing  but  a  caput  mortuum.  The  affinity,  whether  mechan- 
ical or  chemical,  is  still  there  after  the  union  just  as  much  as 


HIGHEST   VELOCITY   OF   A   FALLING   BODY.  337 

before,  and  opposes  a  certain  resistance  to  the  reduction  of 
the  compound ;  but  its  power  of  performance  (Leistungs- 
fahigkeit)  is  at  an  end  as  soon  as  there  is  no  further  available 
falling-space. 

Whenever  the  attraction  becomes  indefinitely  small,  or 
ceases  altogether,  space  is  no  longer  effective  space  ;  and  thus 
it  follows,  from  the  diminution  which  gravity  undergoes  with 
distance,  that  falling-space  is  limited  in  the  centrifugal  direc- 
tion also,  and  hence  that  the  cause  of  motion  or  "  force"  is, 
under  all  circumstances,  a  finite  magnitude  which  becomes 
exhausted  in  producing  its  effect. 

This  fundamental  physical  truth  will  be  most  easily  per- 
ceived when  applied  to  a  special  case  and  reduced  to  figures. 
When  a  pound  weight  is  lifted  one  foot  from  the  ground,  the 
available  force  is,  as  every  one  knows,  =one  foot-pound.  If 
the  falling-height  of  this  weight  amounts  to  n  feet,  n  not  be- 
ing a  large  number,  the  force  may  be  taken  as  approximately 
— n  foot-pounds.  But  supposing  w,  or  the  original  distance 
of  the  weight  from  the  earth,  to  be  very  considerable,  or  in- 
deed infinite,  the  force  (that  is,  the  number  of  foot-pounds) 
does  not  by  any  means  thereby  become  infinite,  but,  according 
to  Newton's  law  of  gravitation,  it  becomes  at  most  =r  foot- 
pounds, where  r  is  the  number  of  feet  contained  in  the  earth's 
semidiameter.  Thus  how  great  soever  the  distance  through 
which  a  weight  falls  against  the  earth,  or  the  time  occupied 
by  its  fall  may  be,  it  can  acquire  no  higher  final  velocity  than 
34,450  Paris  feet  per  second.  On  the  other  hand,  were  the 
mass  of  the  earth  four  times  as  great  as  it  is,  its  bulk  remain- 
ing the  same,  the  force  would  likewise  become  four  times  as 
great,  and  the  maximum  ¥elocity  would  be  68,900  feet. 

It  is  one  of  the  essentials  of  a  good  terminology  that  it 

should  put  fundamental  facts  of  this  kind  in  a  clear  light ; 

exactly  the  opposite,  however,  is  done  by  the  nomenclature  at 

present  in  use.     A  few  expressions,  employed  by  a  very  meri- 

15 


338     THE  MECHANICAL  EQUIVALENT  OF  HEAT. 

torious  naturalist  in  combating  my  views,  may  serve  to  sup- 
port this  assertion. 

"  Although,"  he  says,  "  it  is  quite  true  that  in  nature  no 
motion  can  be  annihilated,  or  that,  as  it  is  commonly  ex- 
pressed, the  quantity  of  motion  once  in  existence  continues 
unceasingly  and  without  any  lessening,  and  although  in  this 
sense  the  character  of  indestructibility  belongs  to  every  prox- 
imate cause  even,  every  primary  cause,  that  is,  every  true 
physical  force,  possesses  the  additional  characteristic  of  being 
inexhaustible.  These  characteristics  will  best  admit  of  being 
unfolded  by  the  closer  consideration  of  gravity,  the  most 
active  and  widely  diffused  of  the  natural  forces  (primary 
causes) ,  which,  as  it  were  the  soul  of  the  world,  indestructi- 
bly and  inexhaustibly  upholds  the  life  of  those  great  masses 
on  whose  motions  depends  the  order  of  the  universe,  while 
requiring  no  food  from  without  to  call  forth  its  ever  renewed 
activity." 

If  these  words  are  intended  to  contain  a  material  contra- 
diction of  the  views  I  have  put  forward,  they  must  be  meant 
to  imply  that,  by  virtue  of  its  being  inexhaustible,  the  attract- 
ive power  of  the  earth  must  be  capable  of  imparting  to  a  fall- 
ing weight,  under  certain  conceivable  circumstances,  an  infin- 
ite velocity.  But  our  author  himself  in  several  places  lets  us 
see  that  he  has  a  (quite  well-founded)  mistrust  of  any  so  de- 
cided a  conclusion :  this  is  shown  in  the  following,  among 
other  passages :  "  If  we  follow  up  the  chain  of  causes  and 
effects  to  its  first  beginnings,  we  come  at  length  to  the  true 
forces  of  nature,  to  those  primary  causes  whose  activity  does 
not  require  that  they  should  be  preceded  by  any  others,  which 
ask  for  no  nourishment,  but  which  can  ever  call  forth  new 
motions,  as  it  were,  out  of  an  inexhaustible  soil,  and  can 
uphold  and  quicken  those  that  are  already  in  being." 

Again :  "If  the  moon  every  moment  falls,  at  least  vir- 
tually, a  certain  distance  toward  the  earth,  what  is  the  force 
which  the  next  moment  pulls  it  away  again,  as  it  were,  in 


OBJECTIONS   CONSIDERED.  339 

order  to  give  rise  to  a  new  falling  force  ?  It  is  precisely  its 
indestructibility  and  inexhaustibility,  its  power  at  all  times 
and  under  all  circumstances  to  bring  about  without  ceasing, 
at  least  virtually,  the  same  effects,  that  is  the  essence  of  every 
true  force  or  primary  cause." 

This  "  as  it  were  "  and  "  at  least  virtually,"  which  always 
slips  in  at  the  critical  moment,  affords  room  for  the  suspicion 
that  our  author  is  himself  not  quite  confident  of  the  power 
of  his  "  true  natural  causes  "  to  give  rise  to  an  inexhaustible 
amount  of  motion  (of  actual  exertion  of  force)  ;  and  the  in- 
definiteness  of  these  expressions  is  quite  characteristic  of  the 
Protean  part  which  the  force  of  gravity  plays  in  writings  on 
natural  science.  The  most  arbitrary  explanations  are  given 
of  this  word,  and  then,  when  facts  no-longer  admit  of  any 
thing  else,  a  retreat  is  sought  in  the  Newtonian  conception. 

Gravity  being  called  a  force,  and  at  the  same  time  the 
term  force  being  connected,  in  accordance  with  the  common 
use  of  language,  with  the  conception  of  an  object  capable  of 
producing  motion,  leads  to  the  false  assumption  that  a  me- 
chanical effect  (the  production  of  motion)  can  be  produced 
without  a  corresponding  expenditure  of  a  measurable  object ; 
and  here  is  likewise  plainly  the  reason  why  our  author  could 
neither  keep  clear  in  his  facts  nor  consistent  in  his  reasoning. 
If  once  the  production  of  motion  out  of  nothing  is  granted, 
the  annihilation  of  motion  must  also  be  admitted  as  a  conse- 
quence ;  and  the  magnitude  of  motion  must,  in  accordance 
with  this  assumption,  be  simply  proportional  to  the  velocity, 
or  =  Jfc,  and  the  "quantity  of  motion  once  in  existence" 
must  be  —-{-Me  —  Jfc^O.  But  notwithstanding  his  "inex- 
haustible forces,"  the  writer  referred  to  expressly  declares 
that  motion  is  indestructible  ;  but,  instead  of  stating  his  opin- 
ion as  to  what  becomes  of  motion  which  disappears  by  fric- 
tion, he  says  in  another  place  again  that  it  remains  "  unde- 
cided" whether  the  effect  of  a  force  (the  amount  of  motion 
produced  by  it)  is.  measured  by  the  first  or  by  the  second 


34:0     THE  MECHANICAL  '  EQUIVALENT  OF  HEAT. 

power  of  the  velocity  (that  is,  whether  it  is  or  is  not  de- 
structible) :  he  even  appears,  from  repeated  expressions,  to 
hold  it  possible  that  a  given  quantity  of  heat  can  produce  mo- 
tion ad  infinitum  I  If  such  were  the  case,  it  would  certainly 
be  useless  to  consider  the  convertibility  of  these  magni- 
tudes :  the  ground  would  rather  have  been  won  for  the  contact 
theory. 

The  polemics  of  my  respected  critic,  whom  I  have  here 
introduced  as  the  representative  and  spokesman  of  prevailing 
views,  and  to  whom  I  feel  that  my  sincere  thanks  are  due  for 
his  attentive  examination  of  my  first  publication,  appear  to 
me  to  be  necessarily  without  result,  inasmuch  as  the  first 
problem  in  combating  my  assertions,  which  all  revolve  about 
the  one  point  of  an  invariable  quantitative  relation  between 
heat  and  motion,  must  be  to  find  out  that  this  relation  is  va- 
riable, and  in  what  cases.  Formal  controversy  without  a 
material  basis  is  only  beating  the  air ;  and  as  to  what  relates 
specially  to  the  questions  about  force,  the  first  point  to  con- 
sider is,  not  what  sort  of  thing  a  "force"  is,  but  to  what 
thing  we  shall  give  the  name  "  force."  Backwards  and  for- 
wards talk  about  gravity  is  fruitless,  since  all  who  understand 
the  matter  are  agreed  as  to  its  nature  ;  for  gravity  is  and  re- 
mains a  differential  quotient  of  the  velocity  by  the  time,  di- 
rectly proportional  to  the  attracting  mass,  and  inversely  pro- 
portional to  the  square  of  the  distance  :  on  this  point  a  final 
decision  was  come  to  long  ago.  But  whether  it  is  expedient 
to  call  this  magnitude  a  force  is  quite  another  question. 

Since,  whenever  an  innovation  of  essential  importance  is 
proposed,  the  public  is  so  ready  to  misapprehend,  I  will  here 
state  once  more,  as  clearly  as  I  can,  my  reasons  for  saving 
that  "  the  force  of  gravity"  is  an  improper  expression. 

It  is  an  unassailable  truth  that  the  production  of  every 
falling  motion  is  connected  with  a  corresponding  expenditure 
of  a  measurable  magnitude.  This  magnitude,  if  it  is  to  be 
made  an  object  of  scientific  investigation  (and  why  should  it 


APPLICATION   OF   THE  TEEM   "FORCE."  341 

not?),  must  have  a  name  given  to  it ;  and  in  accordance  with 
the  logical  instinct  of  man,  as  manifested  in  the  genius  of  lan- 
guage, no  other  name  can  be  here  chosen  than  the  word 
"  force."  But  since  this  expression  is  already  used  in  a  quite 
different  sense,  we  might  be  tempted  to  create  for  the  concep- 
tion which  is  as  yet — in  the  fundamental  parts  of  science  at 
least — unnamed  an  entirely  new  name.  But  before  betaking 
ourselves  to  this  extreme  course,  which  for  reasons  that  are 
not  far  to  seek  would  be  the  one  whereby  we  should  be 
brought  most  into  conflict  with  existing  usage,  it  is  reasonable 
to  inquire  whether  the  word  "  force,"  which' in  itself  answers 
so  well  to  the  requirements  of  the  case,  is  in  its  right  place 
where  it  was  first  put  by  the  schools. 

According  to  the  common  custom  of  speech,  we  under- 
stand by  "  force  "  something  moving — a  cause  of  motion  ;  and 
if,  on  the  one  hand,  the  expression  "  moving  force "  is  for 
this  reason,  strictly  speaking,  a  pleonasm,  the  notion  of  a  not 
moving  or  "  dead"  force  is,  on  the  other  hand,  a  contradidio 
in  adjecto.  If  it  be  said,  for  instance,  that  a  load  which 
presses  with  its  weight  on  the  ground  exerts  thereby  a  force — 
a  force  which,  though  never  so  great,  is  unable  of  itself  to 
bring  about  the  smallest  movement — the  mode  of  conception 
and  of.  expression  is  quite  justified  by  scholastic  usage,  but  it 
is  so  far-fetched  that  it  becomes  the  source  of  unnumbered 
misapprehensions. 

Between  gravity  and  the  force  of  gravity  there  is,  so  far 
as  I  know,  no  difference ;  and  hence  I  consider  the  second 
expression  unscientific,  inasmuch  as  it  is  tautological. 

Let  it  not  be  objected  that  the  "force"  of  pressure,  the 
"  force "  of  gravity,  cohesive  "  force,"  &c.,  are  the  higher 
causes  of  pressure,  gravity,  and  the  like.  The  exact  sciences 
are  concerned  with  phenomena  and  measurable  quantities. 
The  first  cause  of  things  is  Deity — a  Being  ever  inscrutible 
by  the  intellect  of  man  ;  while  "  higher  causes,"  "  supersen- 
suous  forces,"  and  the  rest,  with  all  their  consequences,  be- 


34:2     THE  MECHANICAL  EQUIVALENT  OF  HEAT. 

long  to  the  delusive  middle  region  of  naturalistic  philosophy 
and  mysticism. 

NBy  a  law  that  is  universally  true,  waste  and  want  go  hand 
in  hand.  If  to  the  case  before  us,  where  this  rule  likewise 
meets  with  confirmation,  we  apply  an  equalizing  process,  and 
take  away  the  word  u  force"  from  the  connection  in  which  it 
is  superfluous  and  hurtful,  and  bring  it  to  where  we  are  in 
want  of  it,  we  get  rid  at  one  time  of  two  important  obstacles. 
The  higher  mathematics  at  once  cease  to  be  required  in  order 
to  gain  admittance  into  the  theory  of  motion :  nature  presents 
herself  in  simple  beauty  before  the  astonished  eye,  and  even 
the  less  gifted  may  now  behold  many  things  which  hitherto 
were  concealed  from  the  most  learned  philosophers. 

Force  and  matter  are  indestructible  objects.  This  law,  to 
which  individual  facts  may  most  simply  be  referred,  and 
which  therefore  I  might  figuratively  call  the  heliocentric  stand- 
point, constitutes  a  natural  basis  for  physics,  chemistry,  phys- 
iology, and  philosophy. 

Among  the  facts  which,  though  known,  have  been  hith- 
erto only  empirically  established  and  have  remained  isolated, 
but  which  can  be  easily  referred  to  this  natural  law,  is  the  one 
that  electric  and  magnetic  attraction  cannot  be  isolated  any 
more  than  gravity,  or  that  the  strength  of  this  attraction 
undergoes  no  alteration,  so  long  as  the  distance  remains  the 
same,  by  the  intervening  of  indifferent  substances  (non-con- 
ductors) . 

Among  facts  which  have  remained  unknown  up  to  the 
most  recent  times,  I  will  refer  only  to  the  influence  which  the 
ebb  and  flow  of  the  tide  exerts,  in  accordance  with  the  known 
laws  of  mechanics,  on  the  motion  of  the  earth  about  its  axis. 
A  fact  of  such  importance,  standing,  as  it  does,  in  close  rela- 
tion with  the  fundamental  law  just  stated,  having  been  able 
to  escape  the  attention  of  naturalists,  is  of  itself  a  proof  that 
the  prevailing  system  has  no  exclusive  title. 

For  the  rest,  it  will  not  have  escaped  those  who  are  ac- 


MOTION   AND   FALLING-FOKCE.  343 

quainted  with  the  modern  literature  of  science  that  a  modifi- 
cation of  scientific  language  in  the  sense  of  my  views  is  act- 
ually beginning  to  take  place.  But  in  matters  of  this  kind 
the  chief  part  of  the  work  must  be  left  to  time. 

According  to  what  has  been  said  thus  far,  the  vis  viva  of 
motion  must  be  called  a  force.  But  since  the  expression  vis 
viva  denotes  in  mechanics,  not  only  a  magnitude  which  is 
proportional  to  the  mass  and  to  the  square  of  its  velocity,  but 
also  one  which  is  proportional  to  the  mass  and  to  the  height 
from  which  it  has  fallen,  force  thus  conceived  naturally  di- 
vides itself  into  two  very  easily  distinguished  species,  each  of 
which  requires  a  distinct  technical  name,  for  which  the  words 
motion  (Bewegung)  and  falling-force  (Fallkraft)  seem  to  me 
the  most  appropriate.* 

Hence,  according  to  this  definition,  "  motion "  is  always 
measured  by  the  product  of  the  moved  mass  into  the  square 
of  the  velocity,  never  by  the  product  of  the  mass  into  the 
velocity. 

By  "  falling-force"  we  understand  a  raised  weight,  or  still 
more  generally,  a  distance  in  space  between  two  ponderable 

[*  The  distinction  here  drawn  between  "motion"  and  " falling-force " 
is  the  same  as  that  made  by  Helmholtz  (J)ie  Erhaltung  der  Kraft,  1847) 
between  "  vis  viva  "  (leberidige  Kraft]  and  "  tension  "  (Spankraft).  The  Eng- 
lish expressions  "  dynamical  energy  "  and  "  statical  energy  "  were  used  by 
Prof.  W.  Thomson  (Phil.  Mag.  S.  vol.  iv.  p.  304,  1852)  in  the  same  sense, 
but  were  afterwards  abandoned  by  him  in  favour  of  the  terms  "  actual  en- 
ergy" and  "potential  energy  "  introduced  by  Prof.  Rankine.  More  re- 
cently ("  Good  Words "  for  October,  1862)  Professors  Thomson  and  Tait 
have  employed  the  expression  "kinetic  energy"  in  place  of  "  actual  ener- 
gy." The  German  word  Kraft  in  the  text  has  been  uniformly  translated 
force,  to  which  term  the  ambiguity  of  the  German  original  has  thus  been 
transferred.  This  ambiguity,  however,  may  be  avoided  in  English  by  al- 
lowing the  word  "  force  "  to  retain  the  meaning  which  it  bears  in  common 
language,  that  is,  to  denote  all  resistances  which  it  requires  the  exertion  of 
a  power  to  overcome  (whence  the  expressions  gravitating  force,  cohesive 
force,  &c.),  and  by  using  the  word  "  energy  "  to  denote  force  as  denned  by 
Mayer.— G.  C.  R] 


344  THE  MECHANICAL   EQUIVALENT   OF   HEAT. 

bodies.  In  many  cases  falling-force  is  measured  with  suffi- 
cient accuracy  by  the  product  of  the  raised  weight  into  its 
height  ;  and  the  expressions  "  foot-pound,"  "  kilogramme- 
metre,"  "  horse-power,"  and  many  others,  are  conventional 
units  for  the  measurement  of  this  force,  which  have  of  late 
come  into  general  use,  especially  in  practical  mechanics.  But 
in  order  to  find  the  exact  quantitative  expression  for  the  mag- 
nitude in  question,  we  must  consider  (at  least)  two  masses 
existing  at  a  determinate  distance  from  each  other,  which  ac- 
quire motion  by  mutually  approaching  ;  and  we  must  investi- 
gate the  relation  which  exists  between  the  conditions  of  the 
motion,  namely,  the  magnitude  of  the  masses  and  their  orig- 
inal and  final  distance,  and  the  amount  of  motion  produced. 

It  very  remarkably  happens  that  this  relation  is  the  sim- 
plest conceivable  ;  for,  according  to  Newton's  law  of  gravita- 
tion, the  quantity  of  motion  produced  is  directly  proportional 
to  the  masses  and  to  the  space  through  'which  they  fall,  but 
inversely  proportional  to  the  distances  of  the  centres  of  grav- 
ity of  the  masses  before  and  after  the  movement.  That  is, 
if  A  and  B  are  the  two  masses,  c  and  c'  the  velocities  which 
they  respectively  acquire,  and  h  and  h'  their  original  and  final 
distances  apart,  we  have 


or  in  words,  the  falling-force  is  equal  to  the  product  of  the 
masses  into  the  space  fallen  through  divided  by  the  two  distances. 
By  help  of  this  theorem,  which,  as  will  be  easily  seen,  is 
nothing  but  a  more  general  and  convenient  expression  of 
Newton's  law  of  gravitation,*  the  laws  of  the  fall  of  bodies 

*  Newton's  formula  relates  to  the  particular  case  in  which  the  two  dis- 
tances (the  initial  and  the  final  distance)  are  equal,  so  that  their  product 
becomes  a  square.  In  this  case,  however,  both  the  space  fallen  through 
and  the  velocity  become  nought  ;  and  hence,  when  this  expression  has  to 


OONVERSIONS   OF  MOTION   AND   FALLING-FOECE.       345 

from  cosmical  elevations,  and  also  the  general  laws  of  central 
motions,  can  be  developed  without  its  being  needful  to  employ 
equations  of  more  than  the  second  degree. 

Having  now  become  acquainted  with  two  species  of  force — 
motion  and  falling-force — we  can  arrive  at  a  conception  of  "  a 
force  "  in  general,  according  to  the  well-known  rule,  by  col- 
lecting together  the  common  characteristics  of  the  two  spe- 
cies. To  this  end,  we  must  consider  the  properties  of  these 
objects  somewhat  more  closely.  Their  most  important  prop- 
erty depends  on  their  mutual  relation.  Whenever  a  given 
quantity  of  falling  force  disappears,  motion  is  produced  ;  and 
by  the  expenditure  of  this  latter,  the  falling-force  can  be  re- 
produced in  its  original  amount. 

This  constant  proportion  which  exists  between  falling- 
force  and  motion,  and  is  known  in  the  higher  mechanics  un- 
der the  name  of  "  the  principle  of  the  conservation  of  vis 
viva"  may  be  shortly  and  fitly  denoted  by  the  term  " trans- 
formation "  (  Umwandlung) .  For  instance,  we  may  say  that 
a  planet,  in  passing  from  its  aphelion  to  its  perihelion,  trans- 
forms a  part  of  its  falling-force  into  motion,  and,  as  it  moves 
away  from  the  sun  again,  changes  a  part  of  its  motion  into 
falling-force.  In  using  the  word  "  transform"  in  this  sense, 
nothing  else  can  or  is  intended  to  be  expressed  but  a  constant 
numerical  ratio. 

But  it  follows  from  the  axiom  mentioned  at  page  326,  that 
the  production  of  a  definite  quantity  of  motion  from  a  given 
quantity  of  falling-force,  and  vice  versd,  implies  that  neither 
falling-force  nor  motion  can  be  annihilated  either  totally  or  in 
part.  We  thus  obtain  the  following  definition : 

Forces  are  transformable,  indestructible,  and  (in  contradis- 

be  taken  as  the  starting-point  for  the  calculation  of  real  velocities,  mathe- 
matical artifices  become  necessary  which  are  inadmissible  in  the  elementary 
branches  of  science. 

15* 


346  THE  MECHANICAL   EQUIVALENT  OF   HEAT. 

tinction  from  matter)  imponderable  objects.     (Conf.  paper  al- 
ready quoted,  pp.  328,  329.) 

It  is  easy  to  see  that  this  definition  embraces,  among  other 
things,  the  fact  that  the  motion  which  disappears  in  mechani- 
cal processes  of  different  kinds  bears  a  constant  relation  to 
the  heat  thereby  produced,  or  that  motion  is  convertible,  as 
an  indestructible  magnitude,  into  heat.  Thus  heat  is,  like  mo- 
tion, a  force  ;  and  motion,  like  heat,  an  imponderable. 

I  have  characterized  the  relation  which  various  forces 
bear  to  one  another  by  saying  (Phil.  Mag.  S.  4,  vol.  xxiv.  p. 
252)  that  they  are  "  different  forms  under  which  one  and  the 
same  object  makes  its  appearance."  At  the  same  time  I 
have  expressly  guarded  myself  from  making  the  certainly 
plausible,  but  unproved,  and,  as  it  seems  to  me,  hazardous  de- 
duction that  thermal  phenomena  are  to  be  regarded  as  merely 
phenomena  of  motion.  The  following  is  what  I  said  upon 
this  point  (loc.  cit.)  p.  376  : 

"  But  just  as  little  as  the  connection  between  falling-force 
and  motion  authorizes  the  conclusion  that  the  essence  of  fall- 
ing-force is  motion,  can  such  a  conclusion  be  adopted  in  the 
case  of  heat.  We  are,  on  the  contrary,  rather  inclined  to 
infer  that  before  it  can  become  heat,  motion — whether  simple, 
or  vibratory  as  in  the  case  of  light  and  radiant  heat,  &c. — 
must  cease  to  exist  as  motion." 

The  relation  which,  as  we  have  seen,  subsists  between 
heat  and  motion  has  regard  to  quantity,  not  to  quality ;  for 
(to  borrow  the  words  of  Euclid)  things  which  are  equal  to 
one  another  are  not  therefore  similar.  Let  us  beware  of  leav- 
ing the  solid  ground  of  the  objective,  if  we  would  not  entan- 
gle ourselves  in  difficulties  of  our  own  making. 

In  the  mean  time  it  at  least  results  from  the  foregoing 
considerations  that  the  phenomena  of  heat,  electricity,  and 
magnetism  do  not  owe  their  existence  to  any  particular  fluids  ; 
and  the  immateriality  of  heat,  asserted  half  a  century  ago  by 


VARIOUS   KINDS   OF   HEAT.  347 

Rumford,  becomes,  through  the  discovery  of  its  mechanical 
equivalent,  a  certainty. 

The  form  of  force  denoted  by  the  name  "heat"  is  plainly 
not  single,  but  includes  several  distinct,  though  mutually 
equivalent,  objects,  three  principal  forms  of  which  are  distin- 
guished in  common  language  :  namely,  I.  Radiant  Heat ;  II. 
Free  (sensible)  Heat,  Specific  Heat ;  and  HI.  Latent  Heat. 

There  can  be  no  doubt  that  radiant  heat  must  be  regarded 
as  a  phenomenon  of  motion,  especially  since  the  recent  detec- 
tion of  phenomena  of  interference  in  the  radiation  of  heat. 
But  whether  there  really  exists,  as  is  commonly  assumed,  a 
peculiar  aether,  of  which  the  vibratory  motion  is  perceived  by 
us  as  radiant  heat,  or  whether  the  seat  of  this  motion  is  the 
particles  of  material  bodies,  is  a  question  that  is  not  yet  made 
out. 

Still  greater  obscurity  hangs  about  the  essential  nature  of 
specific  heat,  or  what  goes  on  in  the  interior  of  a  heated  body. 
Not  only  does  the  unanswered  question  of  the  aether  enter 
again  here,  but,  before  we  can  be  in  a  position  to  form  any 
clear  ideas  on  this  subject,  we  require  to  have  an  exact  knowl- 
edge of  the  internal  constitution  of  matter.  We  are,  how- 
ever, still  far  from  having  reached  this  point ;  for,  in  particu- 
lar, we  do  not  know  whether  such  things  as  atoms  exist — that 
is,  whether  matter  consists  of  such  constituents  as  undergo  no 
further  change  of  form  in  chemical  processes. 

But  a  span  of  that  time  which  stretches  both  backwards 
and  forwards  into  eternity  is  meted  out  to  man  here  on  earth, 
and  the  space  which  his  foot  can  tread  is  narrowly  bounded 
above  and  below :  so  also  his  scientific  knowledge  finds  nat- 
ural limits  in  the  direction  of  the  infinitely  small  as  well  as  of 
the  infinitely  great.  The  question  of  atoms  seems  to  me  to 
lead  beyond  these  limits,  and  hence  I  consider  it  unpractical. 
An  atom  in  itself  can  no  more  become  an  object  of  our  inves- 
tigation than  a  differential,  notwithstanding  that  the  ratio 
which  such  immensely  small  auxiliary  magnitudes  bear  to 


348  THE   MECHANICAL   EQUIVALENT  OF   HEAT. 

one  another  may  be  represented  by  concrete  numbers.  In 
every  case,  however,  the  conception  of  an  atom  must  be  re- 
garded as  merely  relative,  and  must  be  considered  in  connec- 
tion with  some  definite  process ;  for,  as  is  well  known,  the 
particles  of  an  acid  and  base  may  play  the  part  of  atoms  in 
the  formation  and  decomposition  of  a  salt,  while  in  another 
process  these  atoms  may  themselves  undergo  further  division. 

But  assuming  that,  in  a  chemical  sense,  atoms  have  a  real 
existence — an  assumption  which,  among  other  things,  the 
laws  of  isomorphism  certainly  render  probable — the  further 
question  arises  whether,  by  the  continued  division  of  matter, 
we  can  at  last  arrive  at  molecules  which  are  atoms  in  relation 
to  the  phenomena  of  heat,  such  that  heat  cannot  penetrate  to 
their  interior,  and  such  that,  when  the  whole  mass  is  heated, 
they  for  their  parts  undergo  no  increase  of  bulk.  But  since 
we  are  unable  to  grapple  with  such  preliminary  questions  as 
hese,  we  are  forced  to  confess  that,  whether  the  existence  of 
«n  a3ther  and  of  atoms  be  admitted  or  not,  we  are,  so  far  as 
^gards  the  nature  of  specific  heat,  in  a  state  of  ignorance. 

The  expression  "latent  heat"  has  reference  to  its  correctly 
•ccognized  property  of  indestructibility.  In  all  cases  in  which 
iiermometrically  sensible  specific  heat  disappears,  it  must  be 
assumed  that  it  eludes  our  perception  only  by  taking  on  some 
other  state  of  existence,  and  that  by  an  appropriate  process 
of  inverse  transformation  the  free  heat  can  be  reproduced  in  its 
original  amount.  These  are  the  facts  on  which  the  doctrine 
of  latent  heat  rests ;  and  hence,  if  we  have  regard  to  them 
only,  all  the  connected  phenomena  may  be  claimed  as  so  many 
confirmations  of  the  principle  of  the  transformation  and  con- 
servation of  force. 

The  conception  of  latent  heat  is  accordingly  nothing  else 
than  the  conception  of  something  equivalent  to  free  heat,  and 
thus  the  doctrine  of  free  and  specific  heat  embraces  pretty 
nearly  the  whole  domain  of  physics.  A  few  examples,  cho- 
sen from  among  the  abundance  of  facts,  may  serve  to  show 


HOW   LATENT   HEAT  IS   TO   BE  REGARDED.  349 

how,  according  to  my  view,  the  phenomena  wherein  heat  be- 
comes latent  are  to  be  regarded. 

If  heat  is  communicated  to  a  gas  retained  under  constant 
pressure,  the  free  heat  of  the  gas  is  increased,  and  at  the 
same  time  a  calculable  quantity  of  heat  becomes  latent;  the 
gas  is  thereby  caused  to  expand,  and  there  is  consequently 
produced  an  amount  of  v is  viva  proportional  to  the  pressure 
and  to  the  space  through  which  expansion  takes  place.  There- 
fore as  soon  as  we  know  how  much  of  the  heat  that  has  be- 
come latent  is  to  be  attributed  to  the  expansion  of  the  gas,  we 
know  also  the  amount  of  the  remainder  of  the  latent  heat 
corresponding  to  the  vis  viva  produced.  Now  Gay-Lussac 
has  proved  by  experiment  that  the  specific  heat  of  a  gas  un- 
dergoes no  sensible  alteration  in  flowing  from  a  containing  ves- 
sel into  a  vacuum.  Hence  it  follows  that  a  gaseous  body  op- 
poses no  perceptible  resistance  to  the  separation  of  its  parti- 
cles, and  that  the  rarefaction  of  a  gas  does  not  of  itself  (that 
is,  when  it  occurs  without  any  evolution  of  force)  cause  any 
heat  to  become  latent.  The  total  quantity  of  heat  which  be- 
comes latent  by  the  expansion  of  a  gas  is  therefore  to  be  taken 
as  the  equivalent  of  the  vis  viva  produced. 

It  results  from  the  principle  of  the  indestructibility  of 
heat — a  principle  which  no  one  calls  in  question — that  the 
quantity  of  heat  which  has  thus  become  latent  must  again 
become  free  when  heat  is  in  any  way  produced  at  the  expense 
of  the  acquired  vis  viva  of  motion.  Motion  is  latent  heat, 
and  heat  is  latent  motion. 

The  celebrated  law  of  Dulong,  that  the  amount  of  heat 
produced  by  the  compression  of  a  gas  is  dependent  on  the 
amount  of  force  expended,  and  not  upon  the  chemical  nature, 
tension,  or  temperature  of  the  gas,  is  a  special  application  of 
the  above  general  principle.  But  in  the  communication  so 
often  mentioned  I  have  shown  that  this  law  of  nature  is  capa- 
ble of  a  very  much  wider  application,  and  that  the  heat  which 
becomes  latent  in  the  expansion  of  a  gas  reappears  again  in 


350  THE   MECHANICAL   EQUIVALENT   OF   HEAT. 

every  case,  if  the  vis  viva  thereby  produced  is  employed  to 
generate  heat,  whether  by  the  compression  of  air,  by  friction, 
or  by  the  impact  of  nonelastic  bodies ;  and  I  have  there 
calculated  the  mechanical  equivalent  of  heat  upon  principles 
of  which  the  accuracy  cannot  be  disputed.  I  also  measured 
at  that  time,  by  way  of  control,  the  heat  produced  in  the 
manufacture  of  paper  in  Holland,  and  compared  it  with  the 
working  force  expended,  and  so  found  a  sufficient  degree  of 
concordance  between  the  two  quantities.  I  have  recently, 
moreover,  succeeded  in  constructing,  for  the  purpose  of  the 
direct  determination  of  the  mechanical  equivalent  of  heat,  a 
very  simple  thermal  dynamometer  on  a  small  scale,  with 
which  the  truth  of  the  principle  in  question  can  be  demon- 
strated ad  oculos ;  and  I  have  reason  to  believe  that  the  effi- 
ciency of  water-wheels  and  steam-engines  might  be  easily  and 
advantageously  measured  by  means  of  a  similar  calorimoto- 
rial  apparatus.  It  must,  however,  be  left  to  the  future  judg- 
ment of  practical  men  to  decide  whether,  and  to  what  extent, 
this  method  deserves  to  be  preferred  to  Prony's. 

Heat  further  becomes  latent  in  certain  changes  of  the  state 
of  aggregation  of  bodies.  Since  it  is  a  settled  fact  that  both 
solid  and  liquid  bodies  oppose  a  certain  resistance  to  the  sep- 
aration of  their  parts,  and  since  in  general  an  expenditure  of 
vis  viva  is  required  for  the  overcoming  of  mechanical  resist- 
ances, we  are  led  to  conclude  d  priori  that  whenever  the  cohe- 
sion of  a  body  is  diminished  or  done  away  with,  force  or  heat 
must  become  latent ;  and  this,  as  is  well  known,  perfectly 
accords  with  experience. 

Starting  from  this  point  of  view,  the  French  physicist 
Person  has  attempted  to  detect  a  direct  quantitative  relation 
between  the  latent  heat  of  metals,  on  which  he  has  made  a 
great  number  of  observations,  and  their  cohesion  ;  but  at  pres- 
ent determinations  of  this  kind  are  beset  with  almost  insur- 
mountable difficulties. 

The  heat  which  becomes  latent  in  the  evaporation  of  water 


HEAT  AND   CHANGES   OF   STATE.  351 

has  been  considered  from  quite  a  similar  point  of  view  by 
Holtzmann  in  his  important  memoir  "  On  the  Heat  and  Elas- 
ticity of  Gases  and  Vapours."  Starting  from  the  principle 
that  elevation  of  temperature  is  equivalent  to  the  raising  of  a 
weight,  this  philosopher  has  likewise  calculated  the  mechani- 
cal equivalent  of  heat  from  the  quantity  of  heat  which  be- 
comes latent  by  the  expansion  of  a  gas  ;  and  he  very  rightly 
conceives  of  the  latent  heat  of  steam  as  made  up  of  two 
parts,  whereof  one,  the  smaller,  is  expended  in  overcoming 
the  opposing  pressure  of  the  atmosphere,  and  can  hence  be 
easily  calculated  by  means  of  the  mechanical  equivalent  of 
heat,  while  the  remaining  part,  the  amount  of  which  can  also 
be  calculated,  is  what  Holtzmann  calls  the  heat  required  to 
destroy  the  cohesion  of  the  water.  In  all  steam-engines  this 
latter  portion  is  wasted,  and  Holtzmann  calculates  from  these 
data  the  superior  efficiency  of  high-pressure  compared  with 
low-pressure  engines.* 

If  the  view  here  taken  of  the  latent  heat  of  fusion  and 
evaporation  is  correct,  heat  must  also  become  latent  when 
hard  bodies  are  reduced  to  powder  ;  and  when  such  substances 
pass  into  the  liquid  condition  from  a  state  of  fine  division, 
they  must  absorb  a  smaller  quantity  of  heat  than  when  they 
are  liquefied  without  previous  comminution.  A  few  experi- 
ments that  I  have  instituted  in  this  direction  have  not  hitherto 
given  any  decisive  result. 

It  is  also  worthy  of  notice  that  certain  solid  bodies  which 
are  capable  of  assuming  allotropic  states,  as,  for  instance,  the 
oxygen-compounds  of  iron,  evolve  a  considerable  quantity  of 
heat  on  passing  from  a  less  to  a  more  hard  condition.  Such 
facts,  the  number  of  which  will  doubtless  continually  increase 
with  time,  agree  perfectly  with  the  above  principle,  that  dim- 
inution of  cohesion  involves  an  expenditure  of  heat,  and,  on 
the  other  hand,  increase  of  cohesion  a  production  of  heat. 

*  The  engines  which  give  the  greatest  useful  effect  must  be  those  in 
which  the  steam  receives  an  addition  of  heat  during  its  expansion. 


352     THE  MECHANICAL  EQUIVALENT  OF  HEAT. 

Customary  language,  according  to  which  gravity  is  called 
a  moving  force  and  heat  a  substance,  occasions,  on  the  one 
hand,  the  significance  of  an  important  natural  object,  falling- 
space,  or  the  space  through  which  a  body  falls,  to  be  kept  as 
much  as  possible  out  of  sight,  and,  on  the  other  hand,  heat  to 
be  removed  to  the  greatest  possible  distance  from  the  vis  viva 
of  motion.  The  sciences  are  thus  reduced  to  an  artificial  sys- 
tem, over  whose  fissured  surface  we  can  advance  in  safety 
only  by  the  powerful  aid  of  the  higher  analysis. 

Without  doubt  the  fact  that  so  simple  and  obvious  a  mat- 
ter as  the  connection  between  heat  and  motion  could  remain 
unperceived  up  to  the  most  recent  times  must  also  be  attrib- 
uted to  the  same  defect.  Nevertheless,  as  has  been  already 
pointed  out,  the  quantitative  determination  of  chemical  heat- 
ing-effects and  of  galvanic  actions,  as  well  as  researches  into 
vital  phenomena,  instituted  in  the  spirit  of  those  of  Liebig, 
must  soon  have  led  to  the  law,  not  difficult  to  discover,  of  the 
equivalence  of  heat  and  motion. 

In  reality  this  law  and  its  numerical  expression,  the  me- 
chanical equivalent  of  heat,  were  published  almost  simulta- 
neously in  Germany  and  in  England. 

Starting  from  the  fact  that  the  amount  of  chemical  as  well 
as  of  galvanic  effect  is  dependent  only  and  solely  on  the 
amount  of  material  expenditure,  the  celebrated  English  phys- 
icist Joule  was  led  to  the  principle  that  the  phenomena  of 
motion  and  of  heat  rest  essentially  upon  one  and  the  same 
foundation,  or,  as  he  expressed  himself,  in  the  same  way  as  I 
have  done,  heat  and  motion  are  transformable  one  into  the 
other. 

Not  only  did  this  philosopher  indisputably  make  an  inde- 
pendent discovery  of  the  natural  law  in  question,  but  to  him 
belongs  the  credit  of  having  made  numerous  and  important 
contributions  towards  its  further  establishment  and  develop- 
ment. Joule  has  shown  that  when  motion  is  produced  by 
means  of  electro-magnetism,  the  heating  effect  of  the  galvanic 


DISCOVERIES   OF  JOULE.  353 

current  is  diminished  in  a  corresponding  and  fixed  proportion. 
He  lias  further  ascertained  that  by  reversing  the  poles  of  a 
magnetic  bar  a  quantity  of  heat  is  produced  proportional  to 
the  square  of  the  magnetic  tension — a  fact  which  was  also 
discovered  by  myself,  though  at  a  later  date.  In  particular, 
Joule  has  likewise  demonstrated,  by  means  of  numerous  ex- 
periments, that  the  heat  evolved  by  friction  under  various  cir- 
cumstances stands  in  an  unvarying  proportion  to  the  amount 
of  force  expended.  According  to  his  most  recent  experiments 
of  this  kind,  he  has  fixed  the  mechanical  equivalent  of  heat 
at  423.* 

Joule  has  likewise  investigated  experimentally,  in  relation 
to  this  question,  the  thermal  behaviour  of  elastic  fluids  when 
expanded,  and  has  thereby  confirmed  the  earlier  results  of 
other  physicists. 

The  new  subject  soon  began  to  excite  the  attention  of 
learned  men  ;  but  inasmuch  as  both  at  home  and  abroad  the 
subject  has  been  exclusively  treated  as  a  foreign  discovery,  I 
find  myself  compelled  to  make  the  claims  to  which  priority  en- 
titles me  ;  for  although  the  few  investigations  which  I  have 
given  to  the  public,  and  which  have  almost  disappeared  in  the 
flood  of  communications  which  every  day  sends  forth  without 
leaving  a  trace  behind,  prove,  by  the  very  form  of  their  pub- 
lication, that  I  am  not  one  who  hankers  after  effect,  it  is  not 
therefore  to  be  assumed  that  I  am  willing  to  be  deprived  of 
intellectual  property  which  documentary  evidence  proves  to 
be  mine. 

By  help  of  the  mechanical  equivalent  of  heat  many  prob- 
lems can  be  solved  which,  without  it,  could  not  be  attacked  at 
all :  among  them,  the  calculation  of  the  thermal  effect  of  the 
falling  together  of  cosmical  masses  may  be  especially  men- 
tioned. It  will  not  be  out  of  place  to  indicate  here  briefly  a 
few  results  of  such  calculations. 

*  That  is,  1  thermal  unit  =-  423  kilogrammetres. 


354     THE  MECHANICAL  EQUIVALENT  OF  HEAT. 

The  following  is  one  problem  of  this  kind.  It  is  assumed 
that  a  cosmical  body  enters  the  atmosphere  of  our  earth  with 
a  velocity  of  four  geographical  miles  per  second,  and  that,  in 
consequence  of  the  resistance  which  it  here  encounters,  it 
loses  so  much  of  its  vis  viva  of  motion  that  its  remaining  ve- 
locity when  it  again  quits  the  atmosphere  amounts  to  three 
miles :  the  question  now  arises,  How  great  is  the  thermal 
effect  which  accompanies  this  process  ? 

A  simple  calculation,  based  upon  the  mechanical  equiva- 
lent of  heat,  shows  that  the  quantity  of  heat  required  is  about 
eight  times  as  great  as  the  heat  of  combustion  of  a  mass  of 
coal  of  equal  weight  with  the  body  in  question,  one  kilo- 
gramme of  coal  being  taken  as  yielding  6,000  thermal  units. 
Hence  it  follows  that  the  velocity  of  the  motion  of  shooting- 
stars  and  fire-balls,  which,  as  is  well-known,  attains,  accord- 
ing to  astronomical  observations,  to  from  four  to  eight  miles, 
is  a  cause  fully  sufficient  to  produce  the  most  violent  evolution 
of  heat,  and  an  insight  into  the  nature  of  these  remarkable 
phenomena  is  thereby  afforded  to  us.* 

The  following  is  a  problem  of  a  similar  kind :  if  two  cos- 
mical masses,  moving  in  space  about  their  common  centre  of 
gravity,  were  by  any  cause  whatever,  for  example  by  the  re- 
sistance of  the  surrounding  medium,  caused  to  fall  together, 
the  question  again  arises,  Ho'w  great  is  the  thermal  effect  cor- 
responding to  this  process  of  mechanical  combination  ? 

Even  though  the  elements  of  the  orbits  (that  is,  their  ex- 
centricity)  may  be  unknown,  we  can  nevertheless  calculate 
from  the  given  weight  and  volume  of  the  masses  in  question 
the  maximum  and  the  minimum  of  the  required  effect.  Thus 
let  it  be  supposed,  for  the  sake  of  an  example,  that  our  earth 
had  been  divided  into  two  equal  globes  which  had  united  in 

*  The  idea  that  the  meteors  here  referred  to  owe  their  light  to  a  me- 
chanical process — whether  friction,  or  the  compression  of  the  air — is  not 
new ;  but  without  a  knowledge  of  the  mechanical  equivalent  of  heat  it 
could  have  no  scientific  foundation. 


COLLISION   OF   COSMICAL   MASSES.  355 

the  manner  described :  calculation  teaches  us  that  the  amount 
of  heat  which  would  have  been  evolved  in  such  a  case  would 
considerably  exceed  that  which  an  equal  weight  of  matter 
could  furnish  by  the  most  intense  process  of  chemical  action. 

It  is  more  than  probable  that  the  earth  has  come  into  ex- 
istence in  some  such  way,  and  that  in  consequence  our  sun,  as 
seen  from  the  distance  of  the  fixed  stars,  exhibited  at  that 
epoch  a  transient  burst  of  light.  But  what  took  place  in  our 
solar  system  perhaps  millions  of  years  ago,  still  goes  on  at 
the  present  time  here  and  there  among  the  fixed  stars  ;  and 
the  transient  appearance  of  stars,  which  in  some  cases,  like 
the  celebrated  star  of  Tycho  Brahe,  have  at  first  an  extraor- 
dinary degree  of  brilliance,  may  be  satisfactorily  explained 
by  assuming  the  falling  together  of  previously  invisible  double 
stars. 

Contrasting  with  such  explosive  bursts  of  light  is  the 
steady  radiation,  shown  continuously  through  enormous  pe- 
riods, by  the  greater  number  of  fixed  stars,  and  among  them 
by  our  sun.  Do  these  appearances,  which  in  so  special  a 
manner  tempt  to  higher  speculations,  constitute  a  real  excep- 
tion to  the  exhaustion  of  a  cause  in  producing  its  effect,  which, 
in  accordance  with  the  foregoing  considerations,  we  have  re- 
garded as  an  established  law  of  Nature  ?  or  does  the  small 
sum  of  human  knowledge  authorize  us  in  supposing  that  here 
also  there  is  an  equivalence  between  performance  and  expen- 
diture, and  in  searching  for  the  conditions  of  that  equivalent  ? 

To  enter  further  upon  this  subject  would  lead  us  beyond 
the  intended  scope  of  this  publication ;  and  I  therefore  close 
in  the  hope  that  the  reader  will  please  to  supplement  by  his 
own  reflection  much  that  in  this  tract  has  been  left  unsaid. 


SOME  THOUGHTS 

ON   THE 

CONSEKYATIOlSr   OF   FOKCE. 

BY  DR.  FARADAY. 


MICHAEL  FARADAY,  son  of  a  smith,  was  born  in  London  in  1791.  He 
was  taught  reading,  writing,  and  arithmetic  at  a  day-school,  and  in  all  other 
things  educated  himself.  At  thirteen  he  was  apprenticed  to  a  bookbinder, 
choosing  this  vocation  in  order  to  be  among  books.  He  was  early  fond  of  ex- 
periment, and  averse  to  trade  ;  and  being  taken  to  hear  some  lectures  of  Sir 
Humphrey  Davy  at  the  Royal  Institution,  he  resolved  to  pursue  science,  and 
wrote  to  Davy  asking  his  assistance  in  obtaining  a  place.  Davy  favored  his 
application,  and  in  1813,  at  the  age  of  twenty,  he  was  appointed  assistant  hi 
the  laboratory  of  the  Royal  Institution.  In  1820  he  discovered  the  chloride 
of  carbon,  and  in  1823  effected  the  condensation  of  chlorine  and  other  gases. 
On  this  account  Davy  became  jealous  of  him,  and  discouraged  the  idea  of 
recommending  him  for  election  to  the  Royal  Society,  which,  however,  took 
place  hi  1824.  In  1820,  Oersted  announced  his  celebrated  discovery  of 
electro-magnetism,  and  Faraday  at  once  entered  upon  an  investigation  of  the 
relations  of  magnetism  and  electricity.  In  1831  he  commenced  his  cele- 
brated series  of  Experimental  Researches  hi  Electricity,  which  extended  to 
three  volumes,  published  in  1839,  1844,  and  1855.  In  1827  he  published 
his  admirable  work  on  "  Chemical  Manipulations,"  and,  in  1830,  a  valuable 
paper  on  "  The  Manufacture  of  Glass  for  Optical  Purposes."  In  1833  he 
became  Professor  of  Chemistry  hi  the  Royal  Institution,  and  he  has  received 
numerous  honors  from  the  learned  societies  of  Europe.  In  1835  he  received 
a  pension  of  £300  a  year,  and  in  1858  the  Queen  allotted  him  a  residence  in 
Hampton  Court.  Dr.  Faraday  has  talents  of  a  high  order,  both  as  an  orig- 
inal investigator  and  as  a  lecturer.  Advanced  in  years,  he  has  now  retired  to 
a  considerable  extent  from  active  duty,  but  is  still  in  the  vigor  of  his  powers, 
as  is  shown  by  his  recent  lectures  to  juvenile  audiences  in  the  Royal  Insti- 
tution. 


THE  CONSERVATION  OF  FOECE. 


~T"T"ARIOUS  circumstances  induce  me  at  the  present  mo- 
V  ment  to  put  forth  a  consideration  regarding  the  con- 
servation of  force.  I  do  not  suppose  that  I  can  utter  any 
.truth  respecting  it  that  has  not  already  presented  itself  to  the 
high  and  piercing  intellects  which  move  within  the  exalted 
regions  of  science ;  but  the  course  of  my  own  investigations 
and  views  makes  me  think  that  the  consideration  may  be  of 
service  to  those  persevering  labourers  (amongst  whom  I  en- 
deavour to  class  myself)  who,  occupied  in  the  comparison  of 
physical  ideas  with  fundamental  principles,  and'  continually 
sustaining  and  aiding  themselves  by  experiment  and  observa- 
tion, delight  to  labour  for  the  advance  of  natural  knowledge, 
and  strive  to  follow  it  into  undiscovered  regions. 

There  is  no  question  which  lies  closer  to  the  root  of  all 
physical  knowledge  than  that  which  inquires  whether  force 
can  be  destroyed  or  not.  The  progress  of  the  strict  science 
of  modern  times  has  tended  more  and  more  to  produce  the 
conviction  that  "force  can  neither  be  created  nor  destroyed ;" 
and  to  render  daily  more  manifest  the  value  of  the  knowledge 
of  that  truth  in  experimental  research.  To  admit,  indeed, 
that  force  may  be  destructible  or  can  altogether  disappear, 
would  be  to  admit  that  matter  could  be  uncreated ;  for  we 
know  matter  only  by  its  forces ;  and  though  one  of  these  is 


360          THE  CONSERVATION  OF  FORCE. 

most  commonly  referred  to,  namely,  gravity,  to  prove  its 
presence,  it  is  not  because  gravity  has  any  pretension,  or  any 
exemption,  amongst  the  forms  of  force  as  regards  the  princi- 
ple of  conservation,  but  simply  that  being,  as  far  as  we  per- 
ceive, inconvertible  in  its  nature  and  unchangeable  in  its  man- 
ifestation, it  offers  an  unchanging  test  of  the  matter  which  we 
recognize  by  it. 

Agreeing  with  those  who  admit  the  conservation  of  force 
to  be  a  principle  in  physics,  as  large  and  sure  as  that  of  the 
indestructibility  of  matter,  or  the  invariability  of  gravity,  I 
think  that  no  particular  idea  of  force  has  a  right  to  unlimited 
or  unqualified  acceptance  that  does  not  include  assent  to  it ; 
and  also,  to  definite  amount  and  definite  disposition  of  the 
force,  either  in  one  effect  or  another,  for  these  are  necessary 
consequences  ;  therefore  I  urge,  that  the  conservation  of  force 
ought  to  be  admitted  as  a  physical  principle  in  all  our  hypoth-  • 
eses,  whether  partial  or  general,  regarding  the  actions  of  mat- 
ter. I  have  had  doubts  in  my  own  mind  whether  the  consid- 
erations I  am  about  to  advance  are  not  rather  metaphysical 
than  physical.  I  am  unable  to  define  what  is  metaphysical  in 
physical  science  ;  and  am  exceedingly  adverse  to  the  easy  and 
unconsidered  admission  of  one  supposition  upon  another,  sug- 
gested as  they  often  are  by  very  imperfect  induction  from  a 
small  number  of  facts,  or  by  a  very  imperfect  observation  of 
the  facts  themselves  ;  but,  on  the  other  hand,  I  think  the  phi- 
losopher may  be  bold  in  his  application  of  principles  which 
have  been  developed  by  close  inquiry,  have  stood  through 
much  investigation,  and  continually  increase  in  force.  For 
instance,  time  is  growing  up  daily  into  importance  as  an  ele- 
ment in  the  exercise  of  force.  The  earth  moves  in  its  orbit 
in  time  ;  the  crust  of  the  earth  moves  in  time  ;  light  movies  in 
time ;  an  electro-magnet  requires  time  for  its  charge  by  an 
electric  current ;  to  inquire,  therefore,  whether  power,  acting 
either  at  sensible  or  insensible  distances,  always  acts  in  time, 
is  not  to  be  metaphysical ;  if  it  acts  in  time  and  across  space, 


ACTION   OF   FORCES   IN   TIME.  361 

it  must  act  by  physical  lines  of  force ;  and  our  view  of  the 
nature  of  the  force  may  be  affected  to  the  extremest  degree 
by  the  conclusions  which  experiment  and  observation  on  time 
may  supply ;  being,  perhaps,  finally  determinable  only  by 
them.  To  inquire  after  the  possible  time  in  which  gravita- 
ting, magnetic,  or  electric  force  is  exerted,  is  no  more  meta- 
physical than  to  mark  the  times  of  the  hands  of  a  clock  in 
their  progress  ;  or  that  of  the  temple  of  Serapis  and  its  ascents 
and  descents ;  or  the  periods  of  the  occultations  of  Jupiter's 
satellites  ;  or  that  in  which  the  light  from  them  comes  to  the 
earth.  Again,  in  some  of  the  known  cases  of  action  in  time, 
'something  happens  whilst  the  time  is  passing  which  did  not 
happen  before,  and  does  not  continue  after ;  it  is,  therefore, 
not  metaphysical  to  expect  an  effect  in  every  case,  or  to  en- 
deavour to  discover  its  existence  and  determine  its  nature. 
So  in  regard  to  the  principle  of  the  conservation  of  force  ;  I 
do  not  think  that  to  admit  it,  and  its  consequences,  whatever 
they  may  be,  is  to  be  metaphysical ;  on  the  contrary,  if  that 
word  have  any  application  to  physics,  then  I  think  that  any 
hypothesis,  whether  of  heat,  or  electricity,  or  gravitation,  or 
any  other  form  of  force,  which  either  willingly  or  unwillingly 
dispenses  with  the  principle  of  conservation,  is  more  liable  to 
the  charge  than  those  which,  by  including  it,  become  so  far 
more  strict  and  precise. 

Supposing  that  the  truth  of  the  principle  of  the  conserva- 
tion of  force  is  assented  to,  I  come  to  its  uses.  No  hypothesis 
should  be  admitted,  nor  any  assertion  of  a  fact  credited,  that 
denies  the  principle.  No  view  should  be  inconsistent  or  in- 
compatible with  it.  Many  of  our  hypotheses  in  the  present 
state  of  science  may  not  comprehend  it,  and  may  be  unable 
to  suggest  its  consequences  ;  but  none  should  oppose  or  con- 
tradict it. 

If  the  principle  be  admitted,  we  perceive  at  once  that  a 
theory  or  definition,  though  it  may  not  contradict  the  princi- 
ple, cannot  be  accepted  as  sufficient  or  complete  unless  the 
16 


362          THE  CONSERVATION  OF  FORCE. 

former  be  contained  in  it ;  that  however  well  or  perfectly  the 
definition  may  include  and  represent  the  state  of  things  com- 
monly considered  under  it,  that  state  or  result  is  only  partial, 
and  must  not  be  accepted  as  exhausting  the  power  or  being 
the  full  equivalent,  and  therefore  cannot  be  considered  as 
representing  its  whole  nature  ;  that,  indeed,  it  may  express  only 
a  very  small  part  of  the  whole,  only  a  residual  phenomenon, 
and  hence  give  us  but  little  indication  of  the  full  natural  truth. 
Allowing  the  principle  its  force,  we  ought,  in  every  hypothe- 
sis, either  to  account  for  its  consequences  by  saying  what  the 
changes  are  when  force  of  a  given  kind  apparently  disappears, 
as  when  ice  thaws,  or  else  should  leave  space  for  the  idea  of 
the  conversion.  If  any  hypothesis,  more  or  less  trustworthy 
on  other  accounts,  is  insufficient  in  expressing  it  or  incompat- 
ible with  it,  the  place  of  deficiency  or  opposition  should  be 
marked  as  the  most  important  for  examination,  for  there  lies 
the  hope  of  a  discovery  of  new  laws  or  a  new  condition  of 
force.  The  deficiency  should  never  be  accepted  as  satisfac- 
tory, but  be  remembered  and  used  as  a  stimulant  to  further 
inquiry ;  for  conversions  of  force  may  here  be  hoped  for. 
Suppositions  may  be  accepted  for  the  time,  provided  they  are 
not  in  contradiction  with  the  principle.  Even  an  increased  or 
diminished  capacity  is  better  than  nothing  at  all,  because  such 
a  supposition,  if  made,  must  be  consistent  with  the  nature  of 
the  original  hypothesis,  and  may,  therefore,  by  the  application 
of  experiment,  be  converted  into  a  further  test  of  probable 
truth.  The  case  of  a  force  simply  removed  or  suspended, 
without  a  transferred  exertion  in  some  other  direction,  appears 
to  me  to  be  absolutely  impossible. 

If  the  principle  be  accepted  as  true,  we  have  a  right  to 
pursue  it  to  its  consequences,  no  matter  what  they  may  be. 
It  is,  indeed,  a  duty  to  do  so.  A  theory  may  be  perfection, 
as  far  as  it  goes,  but  a  consideration  going  beyond  it,  is  not 
for  that  reason  to  be  shut  out.  We  might  as  well  accept  our 
limited  horizon  as  the  limits  of  the  world.  No  magnitude, 


RELATION  OF  GRAVITY.  363 

either  of  the  phenomena  or  of  the  results  to  be  dealt  with, 
should  stop  our  exertions  to  ascertain,  by  the  use  of  the  prin- 
ciple, that  something  remains  to  be  discovered,  and  to  trace 
in  what  direction  that  discovery  may  lie. 

I  will  endeavour  to  illustrate  some  of  the  points  which 
have  been  urged,  by  reference,  in  the  first  instance,  to  a  case 
of  power,  which  has  long  had  great  attractions  for  me,  be- 
cause of  its  extreme  simplicity,  its  promising  nature,  its  uni- 
versal presence,  and  in  its  invariability  under  like  circum- 
stances ;  on  which,  though  I  have  experimented  *  and  as  yet 
failed,  I  think  experiment  would  be  well  bestowed,  I  mean  the 
force  of  gravitation.  I  believe  I  represent  the  received  idea 
of  the  gravitating  force  aright  in  saying  that  it  is  a  simple  at- 
tractive force  exerted  between  any  two  or  all  the  particles  or 
masses  of  matter,  at  every  sensible  distance,  but  with  a  strength 
varying  inversely  as  the  square  of  the  distance.  The  usual 
idea  of  the  force  implies  direct  action  at  a  distance  ;  and  such 
a  view  appears  to  present  little  difficulty  except  to  Newton, 
and  a  few,  including  myself,  who  in  that  respect  may  be  of 
like  mind  with  him. 

This  idea  of  gravity  appears  to  me  to  ignore  entirely  the 
principle  of  the  conservation  of  force ;  and  by  the  terms  of 
its  definition,  if  taken  in  an  absolute  sense,  "  varying  inversely 
as  the  square  of  the  distance,"  to  be  in  direct  opposition  to  it, 
and  it  becomes  my  duty  now  to  point  out  where  this  contra- 
diction occurs,  and  to  use  it  in  illustration  of  the  principle  of 
conservation.  Assume  two  particles  of  matter,  A  and  B,  in 
free  space,  and  a  force  in  each  or  in  both  by  which  they  gravi- 
tate towards  each  other,  the  force  being  unalterable  for  an 
unchanging  distance,  but  varying  inversely  as  the  square  of 
the  distance  when  the  latter  varies.  Then,  at  the  distance  of 
ten,  the  force  may  be  estimated  as  one  ;  whilst  at  the  distance 
of  one,  that  is,  one-tenth  of  the  former,  the  force  will  be  one 

*  Philosophical  Transactions,  1851,  p.  1. 


364          THE  CONSEKVATION  OF  FOECE. 

hundred ;  and  if  we  suppose  an  elastic  spring  to  be  intro- 
duced between  the  two  as  a  measure  of  the  attractive  force, 
the  power  compressing  it  will  be  a  hundred  times  as  much  in 
the  latter  case  as  in  the  former.  But  from  whence  can  this 
enormous  increase  of  power  come  ?  If  we  say  that  it  is  the 
character  of  this  force,  and  content  ourselves  with  that  as  a 
sufficient  answer,  then  it  appears  to  me  we  admit  a  creation 
of  power  and  that  to  an  enormous  amount ;  yet  by  a  change 
of  condition,  so  small  and  simple  as  to  fail  in  leading  the  least 
instructed  mind  to  think  that  it  can  be  a  sufficient  cause,  we 
should  admit  a  result  which  would  equal  the  highest  act  our 
minds  can  appreciate  of  the  working  of  infinite  power  upon 
matter ;  we  should  let  loose  the  highest  law  in  physical  sci- 
ence which  our  faculties  permit  us  to  perceive,  namely,  the 
conservation  of  force.  Suppose  the  two  particles,  A  and  B, 
removed  back  to  the  greater  distance  of  ten,  then  the  force  of 
attraction  would  be  only  a  hundredth  part  of  that  they  pre- 
viously possessed ;  this,  according  to  the  statement  that  the 
force  varies  inversely  as  the  square  of  the  distance,  would 
double  the  strangeness  of  the  above  results ;  it  would  be  an 
annihilation  of  force — an  effect  equal  in  its  infinity  and  its 
consequences  with  creation,  and  only  within  the  power  of  Him 
who  has  created. 

We  have  a  right  to  view  gravitation  under  every  form  that 
either  its  definition  or  its  effects  can  suggest  to  the  mind  ;  it 
is  our  privilege  to  do  so  with  every  force  in  nature  ;  and  it  is 
only  by  so  doing  that  we  have  succeeded,  to  a  large  extent,  in 
relating  the  various  forms  of  power,  so  as  to  derive  one  from 
another,  and  thereby  obtain  confirmatory  evidence  of  the 
great  principle  of  the  conservation  of  force.  Then  let  us 
consider  the  two  particles,  A  and  B,  as  attracting  each  other 
by  the  force  of  gravitation,  under  another  view.  According 
to  the  definition,  the  force  depends  upon  both  particles,  and  if 
the  particle  A  or  B  were  by  itself,  it  could  not  gravitate,  that 
is,  it  could  have  no  attraction,  no  force  of  gravity.  Suppos- 


THE  CASE   OF   GRAVITATING   PARTICLES.  365 

ing  A  to  exist  in  that  isolated  state  and  without  gravitating 
force,  and  then  B  placed  in  relation  to  it,  gravitation  comes 
on,  as  is  supposed,  on  the  part  of  both.  Now,  without  try- 
ing to  imagine  how  B,  which  had  no  gravitating  force,  can 
raise  up  gravitating  force  in  A ;  and  how  A,  equally  without 
force  beforehand,  can  raise  up  force  in  B,  still,  to  imagine  it 
as  a  fact  done,  is  to  admit  a  creation  of  force  in  both  parti- 
cles ;  and  so  to  bring  ourselves  within  the  impossible  conse- 
quences which  have  been  already  referred  to. 

It  may  be  said  we  cannot  have  an  idea  of  one  particle  by 
itself,  and  so  the  reasoning  fails.  For  my  part  I  can  compre- 
hend a  particle  by  itself  just  as  easily  as  many  particles  ;  and 
though  I  cannot  conceive  the  relation  of  a  lone  particle  to 
gravitation,  according  to  the  limited  view  which  is  at  present 
taken  of  that  force,  I  can  conceive  its  relation  to  something 
which  causes  gravitation,  and  with  which,  whether  the  parti- 
cle is  alone,  or  one  of  a  universe  of  other  particles,  it  is  al- 
ways related.  But  the  reasoning  upon  a  lone  particle  does 
not  fail ;  for  as  the  particles  can  be  separated,  we  can  easily 
conceive  of  the  particle  B  being  removed  to  an  infinite  dis- 
tance from  A,  and  then  the  power  in  A  will  be  infinitely  di- 
minished. Such  removal  of  B  will  be  as  if  it  were  annihi- 
lated in  regard  to  A,  and  the  force  in  A  will  be  annihilated 
at  the  same  time  ;  so  that  the  case  of  a  lone  particle  and  that 
where  different  instances  only  are  considered  become  one,  be- 
ing identical  with  each  other  in  their  consequences.  And  as 
removal  of  B  to  an  infinite  distance  is  as  regards  A  annihila- 
tion of  B,  so  removal  to  the  smallest  degree  is,  in  principle, 
the  same  thing  with  displacement  through  infinite  space  ;  the 
smallest  increase  in  distance  involves  annihilation  of  power  ; 
the  annihilation  of  the  second  particle,  so  as  to  have  A  alone, 
involves  no  other  consequence  in  relation  to  gravity ;  there  is 
difference  in  degree,  but  no  difference  in  the  character  of  the 
result. 

It  seems  hardly  necessary  to  observe,  that  the  same  line 


366          THE  CONSERVATION  OF  FORCE. 

of  thought  grows  up  in  the  mind,  if  we  consider  the  mutual 
gravitating  action  of  one  particle  and  many.  The  particle  A 
will  attract  the  particle  B  at  the  distance  of  a  mile  with  a 
certain  degree  of  force ;  it  will  attract  a  particle  C  at  the 
same  distance  of  a  mile  with  a  power  equal  to  that  by  which 
it  attracts  B ;  if  myriads  of  like  particles  be  placed  at  the 
given  distance  of  a  mile,  A  will  attract  each  with  equal  force  ; 
and  if  other  particles  be  accumulated  round  it,  within  and 
without  the  sphere  of  two  miles  diameter,  it  will  attract  them 
all  with  a  force  varying  inversely  with  the  square  of  the  dis- 
tance. How  are  we  to  conceive  of  this  force  growing  up  in 
A  to  a  million-fold  or  more,  and  if  the  surrounding  particles 
be  then  removed,  of  its  diminution  in  an  equal  degree  ?  Or, 
how  are  we  to  look  upon  the  power  raised  up  in  all  these 
outer  particles  by  the  action  of  A  on  them,  or  by  their  action 
one  on  another,  without  admitting,  according  to  the  limited 
definition  of  gravitation,  the  facile  generation  and  annihilation 
of  force? 

The  assumption  which  we  make  for  the  time  with  regard 
to  the  nature  of  a  power  (as  gravity,  heat,  etc.),  and  the 
form  of  words  in  which  we  express  it,  that  is,  its  definition, 
should  be  consistent  with  the  fundamental  principles  of  force 
generally.  The  conservation  of  force  is  a  fundamental  prin- 
ciple ;  hence  the  assumption  with  regard  to  a  particular  form 
of  force  ought  to  imply  what  becomes  of  the  force  when  its 
action  is  increased  or  diminished,  or  its  direction  changed ;  or 
else  the  assumption  should  admit  that  it  is  deficient  on  that 
point,  being  only  half  competent  to  represent  the  force  ;  and, 
in  any  case,  should  not  be  opposed  to  the  principle  of  conser- 
vation. The  usual  definition  of  gravity  as  an  attractive  force 
between  the  particles  of  matter  VARYING  inversely  as  the  square 
of  the  distance,  whilst  it  stands  as  a  full  definition  of  the 
power,  is  inconsistent  with  the  principle  of  the  conservation 
of  force.  If  we  accept  the  principle,  such  a  definition  must 
be  an  imperfect  account  of  the  whole  of  the  force,  and  is 


GRAVITATION   BUT   PARTIALLY   UNDERSTOOD.          367 

probably  only  a  description  of  one  exercise  of  that  power, 
whatever  the  nature  of  the  force  itself  may  be.  If  the  defi- 
nition be  accepted  as  tacitly  including  the  conservation  of 
force,  then  it  ought  to  admit  that  consequences  must  occur 
during  the  suspended  or  diminished  degree  in  its  power  as 
gravitation,  equal  in  importance  to  the  power  suspended  or 
hidden ;  being  in  fact  equivalent  to  that  diminution.  It  ought 
also  to  admit,  that  it  is  incompetent  to  suggest  or  deal  with 
any  of  the  consequences  of  that  changed  part  or  condition  of 
the  force,  and  cannot  tell  whether  they  depend  on,  or  are  re- 
lated to,  conditions  external  or  internal  to  the  gravitating  par- 
ticle ;  and,  as  it  appears  to  me,  can  say  neither  yes  nor  no  to 
any  of  the  arguments  or  probabilities  belonging  to  the  subject. 

If  the  definition  denies  the  occurrence  of  such  contingent 
results,  it  seems  to  me  to  be  unphilosophical ;  if  it  simply  ig- 
nores them,  I  think  it  is  imperfect  and  insufficient ;  if  it  ad- 
mits these  things,  or  any  part  of  them,  then  it  prepares  the 
natural  philosopher  to  look  for  effects  and  conditions  as  yet 
unknown,  and  is  open  to  any  degree  of  development  of  the 
consequences  and  relations  of  power ;  by  denying,  it  opposes 
a  dogmatic  barrier  to  improvement ;  by  ignoring,  it  becomes 
in  many  respects  an  inert  thing,  often  much  in  the  way ;  by 
admitting,  it  rises  to  the  dignity  of  a  stimulus  to  investigation, 
a  pilot  to  human  science. 

The  principle  of  the  conservation  of  force  would  lead  us 
to  assume,  that  when  A  and  B  attract  each  other  less,  be- 
cause of  increasing  distance,  then  some  other  exertion  of 
power,  either  within  or  without  them,  is  proportionately  grow- 
ing up  ;  and  again,  that  when  their  distance  is  diminished,  as 
from  ten  to  one,  the  power  of  attraction,  now  increased  a 
hundred-fold,  has  been  produced  out  of  some  other  form  of 
power  which  has  been  equivalently  reduced.  This  enlarged 
assumption  of  the  nature  of  gravity  is  not  more  metaphysical 
than  the  half  assumption  ;  and  is,  I  believe,  more  philosophi- 
cal and  more  in  accordance  with  all  physical  considerations. 


368          THE  CONSERVATION  OF  FOKOE. 

The  half  assumption  is,  in  my  view  of  the  matter,  more  dog- 
matic and  irrational  than  the  whole,  because  it  leaves  it  to  be 
understood  that  power  can  be  created  and  destroyed  almost  at 
pleasure. 

"When  the  equivalents  of  the  various  forms  of  force,  as  far 
as  they  are  known,  are  considered,  their  differences  appear 
very  great ;  thus,  a  grain  of  water  is  known  to  have  electric 
relations  equivalent  to  a  very  powerful  flash  of  lightning.  It 
may  therefore  be  supposed  that  a  very  large  apparent  amount 
of  the  force  causing  the  phenomena  of  gravitation,  may  be 
the  equivalent  of  a  very  small  change  in  some  unknown  con- 
dition of  the  bodies,  whose  attraction  is  varying  by  change  of 
distance.  For  my  own  part,  many  considerations  urge  my 
mind  toward  the  idea  of  a  cause  of  gravity,  which  is  not  res- 
ident in  the  particles  of  matter  merely,  but  constantly  in 
them,  and  all  space.  I  have  already  put  forth  considerations 
regarding  gravity  which  partake  of  this  idea,*  and  it  seems 
to  have  been  unhesitatingly  accepted  by  Newton.f 

There  is  one  wonderful  condition  of  matter,  perhaps  its 
only  true  indication,  namely,  inertia ;  but  yi  relation  to  the 
ordinary  definition  of  gravity,  it  only  adds  to  the  difficulty. 
For  if  we  consider  two  particles  of  matter  at  a  certain  dis- 
tance apart,  attracting  each  other  under  the  power  of  gravity, 
and  free  to  approach,  they  will  approach  ;  and  when  at  only 
half  the  distance,  each  will  have  had  stored  up  in  it,  because  of 
its  inertia,  a  certain  amount  of  mechanical  force.  This  must 

*  Proceedings  of  the  Royal  Institution,  1855,  vol.  ii.,  p.  10,  etc. 

f  "  That  gravity  should  be  innate,  inherent,  and  essential  to  matter,  so 
that  one  body  may  act  upon  another  at  a  distance,  through  a  vacuum,  with- 
out the  mediation  of  any  thing  else,  by  and  through  which  their  action  and 
force  may  be  conveyed  from  one  to  another,  is  to  me  so  great  an  absurdity 
that  I  believe  no  man  who  has  hi  philosophical  matters  a  competent  fac- 
ulty of  thinking,  can  ever  fall  into  it.  Gravity  must  be  caused  by  an  agent, 
acting  constantly  according  to  certain  laws ;  but  whether  this  agent  be  ma- 
terial or  immaterial  I  have  left  to  the  consideration  of  my  reader." — See 
Newton's  Third  Letter  to  Bentley. 


369 

be  due  to  the  force  exerted,  and,  if  the  conservation  principle 
be  true,  must  have  consumed  an  equivalent  proportion  of  the 
cause  of  attraction ;  and  yet,  according  to  the  definition  of 
gravity,  the  attractive  force  is  not  diminished  thereby,  but 
increased  four-fold,  the  force  growing  up  within  itself  the 
more  rapidly,  the  more  it  is  occupied  in  producing  other  force. 
On  the  other  hand,  if  mechanical  force  from  without  be  used 
to  separate  the  particles  to  twice  their  distance,  this  force  is 
not  stored  up  in  momentum  or  by  inertia,  but  disappears  ;  and 
three-fourths  of  the  attractive  force  at  the  first  distance  disap- 
pears with  it.  How  can  this  be  ? 

We  know  not  the  physical  condition  or  action  from  which 
inertia  results  ;  but  inertia  is  always  a  pure  case  of  the  con- 
servation of  force.  It  has  a  strict  relation  to  gravity,  as  ap- 
pears by  the  proportionate  amount  of  the  force  which  gravity 
can  communicate  to  the  inert  body ;  but  it  appears  to  have 
the  same  strict  relation  to  other  forces  acting  at  a  distance  as 
those  of  magnetism  or  electricity,  when  they  are  so  applied 
by  the  tangential  balance  as  to  act  independent  of  the  gravi- 
tating force.  It  has  the  like  strict  relation  to  force  communi- 
cated by  impact,  pull,  or  in  any  other  way.  It  enables  a 
body  to  take  up  and  conserve  a  given  amount  of  force  until 
that  force  is  transferred  to  other  bodies,  or  changed  into  an 
equivalent  'of  some  other  form ;  that  is  all  that  we  perceive 
in  it ;  and  we  cannot  find  a  more  striking  instance  amongst 
natural,  or  possible  phenomena,  of  the  necessity  of  the  con- 
servation of  force  as  a  law  of  nature ;  or  one  more  in  con- 
trast with  the  assumed  variable  condition  of  the  gravitating 
force  supposed  to  reside  in  the  particles  of  matter 

Even  gravity  itself  furnishes  the  strictest  proof  of  the  con- 
servation of  force  in  this,  that  its  power  is  unchangeable  for 
the  same  distance ;  and  is  by  that  in  striking  contrast  with 
the  variation  which  we  assume  in  regard  to  the  cause  of  grav- 
ity',  to  account  for  the  results  at  different  distances. 

It  will  not  be  imagined  for  a  moment  that  I  am  opposed 
16* 


370          THE  CONSERVATION  OF  FOECE. 

to  what  may  be  called  the  law  of  gravitating  action,  that  is, 
the  law  by  which  all  the  known  effects  of  gravity  are  gov- 
erned ;  what  I  am  considering  is  the  definition  of  the  force  of 
gravitation.  That  the  result  of  one  exercise  of  a  power  may 
be  inversely  as  the  square  of  the  distance,  I  believe  and  ad- 
mit ;  and  I  know  that  it  is  so  in  the  case  of  gravity,  and  has 
been  verified  to  an  extent  that  could  hardly  have  been  within 
the  conception  even  of  Newton  himself  when  he  gave  utter- 
ance to  the  law ;  but  that  the  totality  of  a  force  can  be  em- 
ployed according  to  that  law  I  do  not  believe,  either  in  rela- 
tion to  gravitation,  or  electricity,  or  magnetism,  or  any  other 
supposed  form  of  power. 

I  might  have  drawn  reasons  for  urging  a  continual  recol- 
lection of,  and  reference  to,  the  principle  of  the  conservation 
of  force  from  other  forms  of  power  than  that  of  gravitation  ; 
but  I  think  that  when  founded  on  gravitating  phenomena, 
they  appear  in  their  greatest  simplicity ;  and  precisely  for  this 
reason,  that  gravitation  has  not  yet  been  connected  by  any 
degree  of  convertibility  with  the  other  forms  of  force.  If  I 
refer  for  a  few  minutes  to  these  other  forms,  it  is  only  to 
point  in  their  variations,  to  the  proofs  of  the  value  of  the 
principle  laid  down,  the  consistency  of  the  known  phenomena 
with  it,  and  the  suggestions  of  research  and  discovery  which 
arise  from  it.  Heat,  for  instance,  is  a  mighty  form  of  power, 
and  its  effects  have  been  greatly  developed ;  therefore,  assump- 
tions regarding  its  nature  become  useful  and  necessary,  and 
philosophers  try  to  define  it.  The  most  probable  assumption 
is,  that  it  is  a  motion  of  the  particles  of  matter  ;  but  a  view, 
at  one  time  very  popular,  is,  that  it  consists  of  a  particular 
fluid  of  heat.  Whether  it  be  viewed  in  one  way  or  the  other, 
the  principle  of  conservation  is  admitted,  I  believe,  with  all  its 
force.  When  transferred  from  one  portion  to  another  portion 
of  like  matter,  the  full  amount  of  heat  appears.  When  trans- 
ferred to  matter  of  another  kind  an  apparent  excess  or  defi- 
ciency often  results  ;  the  word  "  capacity"  is  then  introduced, 


USES   OF  THE  PRINCIPLE.  371 

which,  while  it  acknowledges  the  principle  of  conservation, 
leaves  space  for  research.  When  employed  in  changing  the 
state  of  bodies,  the  appearance  and  disappearance  of  the  heat 
is  provided  for  consistently  by  the  assumption  of  enlarged  or 
diminished  motion,  or  else  space  is  left  by  the  term  u  capa- 
city" for  the  partial  views  which  remain  to  be  developed. 
When  converted  into  mechanical  force,  in  the  steam  or  air 
engine,  and  so  brought  into  direct  contact  with  gravity,  being 
then  easily  placed  in  relation  to  it,  still  the  conservation  of 
force  is  fully  respected  and  wonderfully  sustained.  The  con- 
stant amount  of  heat  developed  in  the  whole  of  a  voltaic  cur- 
rent described  by  M.  P.  Favre,*  and  the  present  state  of  the 
knowledge  of  thermo-electricity,  are  again  fine,  partial,  or 
subordinate  illustrations  of  the  principles  of  conservation. 
Even  when  rendered  radiant,  and  for  the  time  giving  no  trace 
or  signs  of  ordinary  heat  action,  the  assumptions  regarding 
its  nature  have  provided  for  the  belief  in  the  conservation  of 
force,  by  admitting  either  that  it  throws  the  ether  into  an 
equivalent  state,  in  sustaining  which  for  the  time  the  power 
is  engaged ;  or  else,  that  the  motion  of  the  particles  of  heat 
is  employed  altogether  in  their  own  transit  from  place  to 
place. 

It  is  true  that  heat  often  becomes  evident  or  insensible  in 
a  manner  unknown  to  us  ;  and  we  have  a  right  to  ask  what 
is  happening  when  the  heat  disappears  in  one  part,  as  of  the 
thermo-voltaic  current,  and  appears  in  another ;  or  when  it 
enlarges  or  changes  the  state  of  bodies ;  or  what  would  hap- 
pen, if  the  heat  being  presented,  such  changes  were  purposely 
opposed.  We  have  a  right  to  ask  these  questions,  but  not  to 
ignore  or  deny  the  conservation  of  force ;  and  one  of  the 
highest  uses  of  the  principle  is  to  suggest  such  inquiries.  Ex- 
plications of  similar  points  are  continually  produced,  and  will 
be  most  abundant  from  the  hands  of  those  who,  not  desiring 

*  Comtes  Rendus  1854,  vol.  xxxix.,  p.  1212. 


872         THE  COKSERVATION  OF  FORCE. 

to  ease  their  labour  by  forgetting  the  principle,  are  ready  to 
admit  it,  either  tacitly,  or,  better  still,  effectively,  being  then 
continually  guided  by  it.  Such  philosophers  believe  that  heat 
must  do  its  equivalent  of  work ;  that  if  in  doing  work  it 
seem  to  disappear,  it  is  still  producing  its  equivalent  effect, 
though  often  in  a  manner  partially  or  totally  unknown  ;  and 
that  if  it  give  rise  to  another  form  of  force  (as  we  imperfectly 
express  it),  that  force  is  equivalent  in  power  to  the  heat  which 
has  disappeared. 

What  is  called  chemical  attraction  affords  equally  instruc- 
tive and  suggestive  considerations  in  relation  to  the  principle 
of  the  conservation  of  force.  The  indestructibility  of  indi- 
vidual matter  is  one  case,  and  a  most  important  one,  of  the 
conservation  of  chemical  force.  A  molecule  has  been  en- 
dowed with  powers  which  give  rise  in  it  to  various  qualities, 
and  these  never  change,  either  in  their  nature  or  amount.  A 
particle  of  oxygen  is  ever  a  particle  of  oxygen — nothing  can 
in  the  least  wear  it.  If  it  enters  into  combination  and  disap- 
pears as  oxygen — if  it  pass  through  a  thousand  combinations, 
animal,  vegetable,  mineral — if  it  lie  hid  for  a  thousand  years 
and  then  be  evolved,  it  is  oxygen  with  its  first  qualities,  nei- 
ther more  nor  less.  It  has  all  its  original  force,  and  only 
that ;  the  amount  of  force  which  it  disengaged  when  hiding 
itself  has  again  to  be  employed  in  a  reverse  direction  when  it 
is  set  at  liberty ;  and  if,  hereafter,  we  should  decompose  oxy- 
gen, and  find  it  compounded  of  other  particles,  we  should  only 
increase  the  strength  of  the  proof  of  the  conservation  of 
force,  for  we  should  have  a  right  to  say  of  these  particles, 
long  as  they  have  been  hidden,  all  that  we  could  say  of  the 
oxygen  itself. 

Again,  the  body  of  facts  included  in  the  theory  of  definite 
proportions,  witnesses  to  the  truth  of  the  conservation  of 
force  ;  and  though  we  know  little  of  the  cause  of  the  change 
of  properties  of  the  acting  and  produced  bodies,  or  how  the 
forces  of  the  former  are  hid  amongst  those  of  the  latter,  we 


CHEMICAL   ACTION   AT  A   DISTANCE.  373 

do  not  for  an  instant  doubt  the  conservation,  but  are  moved 
to  look  for  the  manner  in  which  the  forces  are,  for  the  time, 
disposed,  or  if  they  have  taken  up  another  form  of  force,  to 
search  what  that  form  may  be. 

Even  chemical  action  at  a  distance,  which  is  in  such  an- 
tithetical contrast  with  the  ordinary  exertion  of  chemical  affin- 
ity, since  it  can  produce  effects  miles  away  from  the  particles 
on  which  they  depend,  and  which  are  effectual  only  by  forces 
acting  at  insensible  distances,  still  proves  the  same  thing,  the 
conservation  of  force.  Preparations  can  be  made  for  a  chem- 
ical action  in  the  simple  voltaic  circuit,  but  until  the  circuit 
be  complete  that  action  does  not  occur  ;  yet  in  completing  we 
can  so  arrange  the  circuit,  that  a  distant  chemical  action,  the 
perfect  equivalent  of  the  dominant  chemical  action,  shall  be 
produced ;  and  this  result,  whilst  it  establishes  the  electro- 
chemical equivalent  of  power,  establishes  the  principle  of  the 
conservation  of  force  also,  and  at  the  same  time  suggests 
many  collateral  inquiries  which  have  yet  to  be  made  and 
answered,  before  all  that  concerns  the  conservation  in  this 
case  can  be  understood. 

This  and  other  instances  of  chemical  action  at  a  distance 
carry  our  inquiring  thoughts  on  from  the  facts  to  the  physical 
mode  of  the  exertion  of  force  ;  for  the  qualities  which  seem 
located  and  fixed  to  certain  particles  of  matter  appear  at  a 
distance  in  connection  with  particles  altogether  different. 
They  also  lead  our  thoughts  to  the  conversion  of  one  form  of 
power  into  another ;  as,  for  instance,  in  the  heat  which  the 
elements  of  a  voltaic  pile  may  either  show  at  the  place  where 
they  act  by  their  combustion  or  combination  together,  or  in 
the  distance,  where  the  electric  spark  may  be  rendered  mani- 
fest ;  or  in  the  wire  of  fluids  of  the  different  parts  of  the 
circuit. 

When  we  occupy  ourselves  with  the  dual  forms  of  power, 
electricity,  and  magnetism,  we  find  great  latitude  of  assump- 
tion, and  necessarily  so,  for  the  powers  become  more  and 


374          THE  CONSERVATION  OF  FORCE. 

more  complicated  in  their  conditions.  But  still  there  is  no 
apparent  desire  to  let  loose  the  force  of  the  principle  of  con- 
servation, even  in  those  cases  where  the  appearance  and  dis- 
appearance of  force  may  seem  most  evident  and  striking. 
Electricity  appears  when  there  is  consumption  of  no  other 
force  than  that  required  for  friction ;  we  do  not  know  how, 
but  we  search  to  know,  not  being  willing  to  admit  that  the 
electric  force  can  arise  out  of  nothing.  The  two  electricities 
are  developed  in  equal  proportions  ;  and  having  appeared,  we 
may  dispose  variously  of  the  influence  of  one  upon  successive 
portions  of  the  other,  causing  many  changes  in  relation,  yet 
never  able  to  make  the  sum  of  the  force  of  one  kind  in  the 
least  degree  exceed  or  come  short  of  the  sum  of  the  other. 
In  that  necessity  of  equality,  we  see  another  direct  proof  of 
the  conservation  of  force,  in  the  midst  of  a  thousand  changes 
that  require  to  be  developed  in  their  principles  before  we 
can  consider  this  part  of  science  as  even  moderately  known 
to  us. 

One  assumption  with  regard  to  electricity  is,  that  there  is 
an  electric  fluid  rendered  evident  by  excitement  in  plus  and 
minus  proportions.  Another  assumption  is,  that  there  are 
two  fluids  of  electricity,  each  particle  of  each  repelling  all 
particles  like  itself,  and  attracting  all  particles  of  the  other 
kind  always,  and  with  a  force  proportionate  to  the  inverse 
square  of  the  distance,  being  so  far  analogous  to  the  defini- 
tion of  gravity.  This  hypothesis  is  antagonistic  to  the  law 
of  the  conservation  of  force,  and  open  to  all  the  objections 
that  have  been,  or  may  be,  made  against  the  ordinary  defini- 
tion of  gravity.  Another  assumption  is,  that  each  particle  of 
the  two  electricities  has  a  given  amount  of  power,  and  can 
only  attract  contrary  particles  with  the  sum  of  that  amount, 
acting  upon  each  of  two  with  only  half  the  power  it  Could  in 
like  circumstances  exert  upon  one.  But  various  as  are  the 
assumptions,  the  conservation  of  force  (though  wanting  in  the 
second)  is,  I  think,  intended  to  be  included  in  all.  I  might 


OPENING-   OF  NEW   PROBLEMS.  375 

repeat  the  same  observations  nearly  in  regard  to  magnetism — 
whether  to  be  assumed  as  a  fluid,  or  two  fluids  or  electric  cur- 
rents— whether  the  external  action  be  supposed  to  be  action 
at  a  distance,  or  dependent  on  an  external  condition  and 
lines  of  force — still,  all  are  intended  to  admit  the  conserva- 
tion of  power  as  a  principle  to  which  the  phenomena  are 
subject. 

The  principles  of  physical  knowledge  are  now  so  far  de- 
veloped as  to  enable  us  not  merely  to  define  or  describe  the 
known,  but  to  state  reasonable  expectations  regarding  the 
unknown ;  and  I  think  the  principle  of  the  conservation  of 
force  may  greatly  aid  experimental  philosophers  in  that  duty 
to  science,  which  consists  in  the  enunciation  of  problems  to 
be  solved.  It  will  lead  us,  in  any  case  where  the  force  re- 
maining unchanged  in  form  is  altered  in  direction  only,  to 
look  for  the  new  disposition  of  the  force  ;  as  in  the  cases  of 
magnetism,  static  electricity,  and  perhaps  gravity,  and  to  as- 
certain that  as  a  whole  it  remains  unchanged  in  amount— or, 
if  the  original  force  disappear,  either  altogether  or  in  part,  it 
will  lead  us  to  look  for  the  new  condition  or  form  of  force 
which  should  result,  and  to  develop  its  equivalency  to  the 
force  that  has  disappeared.  Likewise,  when  force  is  devel- 
oped, it  will  cause  us  to  consider  the  previously-existing  equiv- 
alent to  the  force  so  appearing ;  and  many  such  cases  there 
are  in  chemical  action.  When  force  disappears,  as  in  the 
electric  or  magnetic  induction  after  more  or  less  discharge,  or 
that  of  gravity  with  an  increasing  distance,  it  will  suggest  a 
research  as  to  whether  the  equivalent  change  is  one  within 
the  apparently  acting  bodies,  or  one  external  (in  part)  to 
them.  It  will  also  raise  up  inquiry  as  to  the  nature  of  the 
internal  or  external  state,  both  before  the  change  and  after. 
If  supposed  to  be  external,  it  will  suggest  the  necessity  of  a 
physical  process,  by  which  the  power  is  communicated  from 
body  to  body ;  and  in  the  case  of  external  action,  will  lead 
to  the  inquiry  whether,  in  any  case,  there  can  be  truly  action 


376          THE  CONSEEVATION  OF  FOKCE. 

at  a  distance,  or  whether  the  ether,  or  some  other  medium,  is 
not  necessarily  present. 

We  are  not  permitted  as  yet  to  see  the  nature  of  the  source 
of  physical  power,  but  we  are  allowed  to  see  much  of  the 
consistency  existing  amongst  the  various  forms  in  which  it  is 
presented  to  us.  Thus,  if,  in  static  electricity,  we  consider 
an  act  of  induction,  we  can  perceive  the  consistency  of  all 
other  like  acts  of  induction  with  it.  If  we  then  take  an  elec- 
tric current,  and  compare  it  with  this  inductive  effect,  we  see 
their  relation  and  consistency.  In  the  same  manner  we  have 
arrived  at  a  knowledge  of  the  consistency  of  magnetism  with 
electricity,  and  also  of  chemical  action  and  of  heat  with  all 
the  former  ;  and  if  we  see  not  the  consistency  between  gravi- 
tation with  any  of  these  forms  of  force,  I  am  strongly  of  the 
mind  that  it  is  because  of  our  ignorance  only.  How  imper- 
fect would  our  idea  of  an  electric  current  now  be,  if  we  were 
to  leave  out  of  sight  its  origin,  its  static  and  dynamic  induc- 
tion, its  magnetic  influence,  its  chemical  and  heating  effects  ; 
or  our  idea  of  any  one  of  these  results,  if  we  left  any  of  the 
others  unregarded  ?  That  there  should  be  a  power  of  gravita- 
tion existing  by  itself,  having  no  relation  to  the  other  natural 
powers,  and  no  respect  to  the  law  of  the  conservation  of  force, 
is  as  little  likely  as  that  there  should  be  a  principle  of  levity 
as  well  as  of  gravity.  Gravity  may  be  only  the  residual  part 
of  the  other  forces  of  nature,  as  Mositi  has  tried  to  show ; 
but  that  it  should  fall  out  from  the  law  of  all  other  force,  and 
should  be  outside  the  reach  either  of  further  experiment  or 
philosophical  conclusions,  is  not  probable.  So  we  must  strive 
"to  learn  more  of  this  outstanding  power,  and  endeavour  to 
avoid  any  definition  of  it  which  is  incompatible  with  the  prin- 
ciples of  force  generally,  for  all  the  phenomena  of  nature  lead 
us  to  believe  that  the  great  and  governing  law  is  one.  I 
would  much  rather  incline  to  believe  that  bodies  affecting 
each  other  by  gravitation  act  by  lines  of  force  of  definite 
amount  (somewhat  in  the  manner  of  magnetic  or  electric  in- 


MENTAL   QUALIFICATIONS  FOE  THE  INQUIRY.        317 

duction,  though,  with  polarity),  or  by  an  ether  pervading  all 
parts  of  space,  than  admit  that  the  conservation  of  force 
could  be  dispensed  with. 

It  may  be  supposed,  that  one  who  has  little  or  no  mathe- 
matical knowledge  should  hardly  assume  a  right  to  judge  of 
the  generality  and  force  of  a  principle  such  as  that  which 
forms  the  subject  of  these  remarks.  My  apology  is  this  :  I 
do  not  perceive  that  a  mathematical  mind,  simply  as  such,  has 
any  advantage  over  an  equally  acute  mind  not  mathematical, 
in  perceiving  the  nature  and  power  of  a  natural  principle  of 
action.  It  cannot  of  itself  introduce  •  the  knowledge  of  any 
new  principle.  Dealing  with  any  and  every  amount  of  static 
electricity,  the  mathematical  mind  has  balanced  and  adjusted 
them  with  wonderful  advantage,  and  has  foretold  results 
which  the  experimentalist  can  do  no  more  than  verify.  But 
it  could  not  discover  dynamic-electricity,  nor  electro-magnet- 
ism, nor  magneto-electricity,  or  even  suggest  them ;  though 
when  once  discovered  by  the  experimentalist,  it  can  take  them 
up  with  extreme  facility. 

So  in  respect  of  the  force  of  gravitation,  it  has  calculated 
the  results  of  the  power  in  such  a  wonderful  manner  as  to 
trace  the  known  planets  through  their  courses  and  perturba- 
tions, and  in  so  doing  has  discovered  a  planet  before  unknown  ; 
but  there  may  be  results  of  the  gravitating  force  of  other 
kinds  than  attraction  inversely  as  the  square  of  the  distance, 
of  which  it  knows  nothing,  can  discover  nothing,  and  can 
neither  assert  nor  deny  their  possibility  or  occurrence.  Under 
these  circumstances,  a  principle  which  may  be  accepted  as 
equally  strict  with  mathematical  knowledge,  comprehensible 
without  it,  applicable  by  all  in  their  philosophical  logic,  what- 
ever form  that  may  take,  and  above  all,  suggestive,  encour- 
aging, and  instructive  to  the  mind  of  the  experimentalist, 
should  be  the  more  earnestly  employed  and  the  more  fre- 
quently resorted  to  when  we  are  labouring  either  to  discover 
new  regions  of  science,  or  to  map  out  and  develop  those 


378          THE  CONSEEVATION  OF  FOKCE. 

which  are  known  into  one  harmonious  whole  ;  and  if  in  such 
strivings,  we,  whilst  applying  the  principle  of  conservation, 
see  but  imperfectly,  still  we  should  endeavour  to  see,  for  even 
an  obscure  and  distorted  vision  is  better  than  none.  Let  us, 
if  we  can,  discover  a  new  thing  in  any  shape ;  the  true  ap- 
pearance and  character  will  be  easily  developed  afterwards. 

Some  are  much  surprised  that  I  should,  as  they  think, 
venture  to  oppose  the  conclusions  of  Newton ;  but  here  there 
is  a  mistake.  I  do  not  oppose  Newton  on  any  point ;  it  is 
rather  those  who  sustain  the  idea  of  action  at  a  distance,  that 
contradict  him.  Doubtful  as  I  ought  to  be  of  myself,  I  am 
certainly  very  glad  to  feel  that  my  convictions  are  in  accord- 
ance with  his  conclusions.  At  the  same  time,  those  who  oc- 
cupy themselves  with  such  matters  ought  not  to  depend  alto- 
gether upon  authority,  but  should  find  reason  within  them- 
selves, after  careful  thought  and  consideration,  to  use  and 
abide  by  their  own  judgment.  Newton  himself,  whilst  refer- 
ring to  those  who  were  judging  his  views,  speaks  of  such  as 
are  competent  to  form  an  opinion  in  such  matters,  and  makes 
a  strong  distinction  between  them  and  those  who  were  incom- 
petent for  the  case. 

But  after  all,  the  principle  of  the  conservation  of  force 
may  by  some  be  denied.  Well,  then,  if  it  be  unfounded  even 
in  its  application  to  the  smallest  part  of  the  science  of  force, 
the  proof  must  be  within  our  reach,  for  all  physical  science 
is  so.  In  that  case,  discoveries  as  large  or  larger  than  any 
yet  made,  may  be  anticipated.  I  do  not  resist  the  search  for 
them,  for  no  one  can  do  harm,  but  only  g'ood,  who  works 
with  an  earnest  and  truthful  spirit  in  such  a  direction.  But 
let  us  not  admit  the  destruction  or  creation  of  force  without 
clear  and  constant  proof.  Just  as  the  chemist  owes  all  the 
perfection  of  his  science  to  his  dependence  on  the  certainty 
of  gravitation  applied  by  the  balance,  so  may  the  physical 
philosopher  expect  to  find  the  greatest  security  and  the  utmost 
aid  in  the  principle  of  the  conservation  of  foice.  All  that 


SUPPLEMENTARY   CONSIDERATIONS.  379 

we  have  that  is  good  and  safe,  as  the  steam-engine,  the  elec- 
tric-telegraph, &c.,  witness  to  that  principle — it  would  require 
a  perpetual  motion,  a  fire  without  heat,  heat  without  a  source, 
action  without  reaction,  cause  with  effect,  or  effect  without  a 
cause,  to  displace  it  from  its  rank  as  a  law  of  nature. 


During  the  year  that  has  passed  since  the  publication  of 
the  foregoing  views  regarding  gravitation,  &c.,  I  have  come  to 
the  knowledge  of  various  observations  upon  them,  some 
adverse,  others  favourable  :  these  have  given  me  no  reason  to 
change  my  own  mode  of  viewing  the  subject ;  but  some  of 
them  make  me  think  that  I  have  not  stated  the  matter  with 
sufficient  precision.  The  word  "force "is  understood  by 
many  to  mean  simply  "  the  tendency  of  a  body  to  pass  from 
one  place  to  another,"  which  is  equivalent,  I  suppose,  to  the 
phrase  "  mechanical  force  ;  "  those  who  so  restrain  its  mean- 
ing must  have  found  my  argument  very  obscure.  What  I 
mean  by  the  word  "  force,"  is  the  cause  of  a  physical  action  ; 
the  source  or  sources  of  all  possible  changes  amongst  the 
particles  or  materials  of  the  universe. 

It  seems  to  me  that  the  idea  of  the  conservation  of  force  is 
absolutely  independent  of  any  notion  we  may  form  of  the 
nature  of  force  or  its  varieties,  and  is  as  sure  and  may  be  as 
firmly  held  in  the  mind,  as  if  we,  instead  of  being  very 
ignorant,  understood  perfectly  every  point  about  the  cause  of 
force  and  the  varied  effects  it  can  produce.  There  may  be 
perfectly  distinct  and  separate  causes  of  what  are  called 
chemical  actions,  or  electrical  actions,  or  gravitating  actions, 
constituting  so  many  forces ;  but  if  the  "  conservation  of 
force  "  is  a  good  and  true  principle,  each  of  these  forces  must 
be  subject  to  it :  none  can  vary  in  its  absolute  amount ;  each 
must  be  definite  at  all  times,  whether  for  a  particle,  or  for  all 
the  particles  in  the  universe ;  and  the  sum  also  of  the  three 


380          THE  CONSERVATION  OF  FOBCE. 

forces  must  be  equally  unchangeable.  Or,  there  may  be  but 
one  cause  for  these  three  sets  of  actions,  and  in  place  of  three 
forces  we  may  really  have  but  one,  convertible  in  its  manifes- 
tations ;  then  the  proportions  between  one  set  of  actions  and 
another,  as  the  chemical  and  the  electrical,  may  become  very 
variable,  so  as  to  be  utterly  inconsistent  with  the  idea  of  the 
conservation  of  two  separate  forces  (the  electrical  and 
the.  chemical),  but  perfectly  consistent  with  the  conservation 
of  a  force,  being  the  common  cause  of  the  two  or  more  sets 
of  action. 

It  is  perfectly  true  that  we  cannot  always  trace  a  force  by 
its  actions,  though  we  admit  its  conservation.  Oxygen  and 
hydrogen  may  remain  mixed  for  years  without .  showing  any 
signs  of  chemical  activity ;  they  may  be  made  at  any  given 
instant  to  exhibit  active  results,  and  then  assume  a  new 
state,  in  which  again  they  appear  as  passive  bodies.  Now, 
though  we  cannot  clearly  explain  what  the  chemical  force  is 
doing,  that  is  to  say,  what  are  its  effects  during  the  three 
periods  before,  at,  and  after  the  active  combination,1  and  only 
by  very  vague  assumption  can  approach  to  a  feeble  concep- 
tion of  its  respective  states,  yet  we  do  not  suppose  the  creation 
of  a  new  portion  of  force  for  the  active  moment  of  time,  or 
the  less  believe  that  the  forces  belonging  to  the  oxygen  and 
hydrogen  exist  unchanged  in  their  amount  at  all  these  periods, 
though  varying  in  their  results.  A  part  may  at  the  active 
moment  be  thrown  off  as  mechanical  force,  a  part  as  radiant 
force,  a  part  disposed  of  we  know  not  how  ;  but  believing,  by 
the  principle  of  conservation,  that  it  is  not  increased  or 
destroyed,  our  thoughts  are  directed  to  search  out  what 
at  all  and  every  period  it  is  doing,  and  how  it  is  to  be 
recognized  and  measured.  A  problem,  founded  on  the 
physical  truth  of  nature,  is  stated,  and,  being  stated,  is  on  the 
way  to  its  solution. 

Those  who  admit  the  possibility  of  the  common  origin  of 
all  physical  force,  and  also  acknowledge  the  principle  of  con- 


SUPEEMACY   OF   THE   PEINCIPLE.  381 

servation,  apply  that  principle  to  the  sum  total  of  the  force. 
Though  the  amount  of  mechanical  force  (using  habitual  lan- 
guage for  convenience  sake)  may  remain  unchanged  and 
definite  in  its  character  for  a  long  time,  yet  when,  as  in  the 
collision  of  two  equal  inelastic  bodies,  it  appears  to  be  lost, 
they  find  it  in  the  form  of  heat ;  and  whether  they  admit  that 
heat  to  be  a  continued  mechanical  action  (as  is  most  proba- 
ble), or  assume  some  other  idea,  as  that  of  electricity,  or 
action  of  a  heat-fluid,  still  they  hold  to  the  principle  of  con- 
servation by  admitting  that  the  sum  of  force,  i.  e.  of  the 
"  cause  of  action,"  is  the  same,  whatever  character  the  effects 
assume.  With  them  the  convertibility  of  heat,  electricity, 
magnetism,  chemical  action  and  motion,  is  a  familiar  thought ; 
neither  can  I  perceive  any  reason  why  they  should  be  led  to 
exclude,  a  priori,  the  cause  of  gravitation  from  association 
with  the  cause  of  these  other  phenomena  respectively.  All 
that  they  are  limited  by  in  their  various  investigations, 
whatever  directions  they  may  take,  is  the  necessity  of  mak- 
ing no  assumption  directly  contradictory  of  the  conservation 
of  force  applied  to  the  sum  of  all  the  forces  concerned,  and  to 
endeavour  to  discover  the  different  directions  in  which  the 
various  parts  of  the  total  force  have  been  exerted. 

Those  who  admit  separate  forces  inter-unchangeable, 
have  to  show  that  each  of  these  forces  is  separately  subject 
to  the  principle  of  conservation.  If  gravitation  be  such  a 
separate  force,  and  yet  its  power  in  the  action  of  two  par- 
ticles be  supposed  to  be  diminished  fourfold  by  doubling  the 
distance,  surely  some  new  action,  having  true  gravitation 
character,  and  that  alone,  ought  to  appear,  for  how  else  can 
the  totality  of  the  force  remain  unchanged?  To  define  the 
force  as  "  a  simple  attractive  force  exerted  between  any  two 
or  all  the  particles  of  matter,  with*  a  strength  varying  in- 
versely as  the  square  of  the  distance,"  is  not  to  answer  the 
question ;  nor  does  it  indicate  or  even  assume  what  are  the 
other  complementary  results  which  occur  ;  or  allow  the  sup- 


382          THE  CONSERVATION  OF  FORCE. 

position  that  such  are  necessary  :  it  is  simply,  as  it  appears 
to  me,  to  deny  the  conservation  of  force. 

As  to  the  gravitating  force,  I  do  not  presume  to  say  that  I 
have  the  least  idea  of  what  occurs  in  two  particles  when  their 
power  of  mutually  approaching  each  other  is  changed  by 
their  being  placed  at  different  distances  ;  but  I  have  a  strong 
conviction,  through  the  influence  on  my  mind  of  the  doctrine 
of  conservation,  that  there  is  a  change ;  and  that  the  phe- 
nomena resulting  from  the  change  will  probably  appear  some 
day  as  the  result  of  careful  research.  If  it  be  said  that 
"  'twere  to  consider  too  curiously  to  consider  so,"  then  I 
must  dissent :  to  refrain  to  consider  would  be  to  ignore  the 
principle  of  the  conservation  of  force,  and  to  stop  the  inquiry 
which  it  suggests — whereas  to  admit  the  proper  logical  force 
of  the  principle  in  our  hypotheses  and  considerations,  and  to 
permit  its  guidance  in  a  cautious  yet  courageous  course  of  in- 
vestigation, may  give  us  power  to  enlarge  the  generalities  we 
already  possess  in  respect  of  heat,  motion,  electricity,  mag- 
netism, &c.,  to  associate  gravity  with  them,  and  perhaps 
enable  us  to  know  whether  the  essential  force  of  gravitation 
(and  other  attractions)  is  internal  or  external  as  respects  the 
attracted  bodies. 

Returning  once  more  to  the  definition  of  the  gravitating 
power  as  "  a  simple  attractive  force  exerted  between  any  two  or 
all  the  particles  or  masses  of  matter  at  every  sensible  distance, 
but  with  a  STRENGTH  VARYING  inversely  as  the  square  of  the 
distance"  I  ought  perhaps  to  suppose  there  are  many  who 
accept  this  as  a  true  and  sufficient  description  of  the  force,  and 
who  therefore ,  in  relation  to  it,  deny  the  principle  of  conser- 
vation. If  both  are  accepted  and  are  thought  to  be  consist- 
ent with  each  other,  it  cannot  be  difficult  to  add  words 
which  shall  make  "varying  strength"  and  "conservation" 
agree  together.  It  cannot  be  said  that  the  definition  merely 
applies  to  the  effects  of  gravitation  as  far  as  we  know  them. 
So  understood,  it  would  form  no  barrier  to  progress ;  for, 


DEFINITION   OF  THE   GRAVITATING   POWER.  383 

that  particles  at  different  distances  are  urged  toward  each 
other  with  a  power  varying  inversely  as  the  square  of  the 
distance,  is  a  truth :  but  the  definition  has  not  that  mean- 
ing; and  what  I  object  to  is  the  pretence  of  knowledge 
which  the  definition  sets  up  when  it  assumes  to  describe,  not 
the  partial  effects  of  the  force,  but  the  nature  of  the  force 
as  a  whole. 


ON 


THE  CONFECTION  AND  EQUIVALENCE 
OF  FOECES 

BY  PROF.  J.  VON  LIEBIG. 


17 


JUSTUS  VON  LIEBIG,  born  at  Darmstadt  in  1803,  after  spending  ten 
months  in  an  apothecary's  shop,  entered  the  University  of  Bonn  in  1819, 
and  afterwards  graduated  in  medicine  in  Erlangen.  In  1822  he  went  to 
Paris,  where  he  studied  chemistry  two  years.  In  1824  he  read  a  paper  on 
the  Fulminates  before  the  French  Institute,  which  attracted  the  attention  of 
Humboldt,  by  whose  influence  he  was  appointed  adjunct  Professor  of  Chem- 
istry in  the  University  of  Geissen.  He  became  professor  of  this  institution 
in  1826,  and  established  here  the  first  laboratory  in  Germany  for-  teaching 
practical  chemistry.  In  1840  he  published  his  "  Chemistry  in  its  applica- 
tions to  Agriculture  and  Physiology,"  in  the  form  of  a  report  to  the  British 
Association.  In  1842  he  reported  to  the  same  body  his  work  on  "Animal 
Chemistry."  About  the  same  time  appeared  his  "Familiar  Letters  on 
Chemistry,"  which  has  since  been  rewritten  and  much  extended.  He  is  the 
author  also  of  various  other  valuable  works.  He  remained  at  Geissen  till 
1852,  when  he  became  professor  and  president  of  the  laboratory  in  the 
University  of  Munich.  In  1854  his  friends  in  Europe  contributed  and  pre- 
sented to  him  £1,000,  and  in  1860  he  became  President  of  the  Academy  of 
Sciences  in  Munich.  Professor  Liebig  is  a  bold  and  intrepid  investigator, 
and  an  ardent  writer,  who  has  made  a  profound  impression  upon  his  age. 
While  some  of  his  views  have  not  been  accepted  hi  the  chemical  world,  and 
indeed  have  been  abandoned  by  himself,  others  have  taken  their  place  as 
valuable  additions  to  the  body  of  scientific  truth.  The  charge  that  some  of 
his  doctrines  have  proved  erroneous  does  not  disturb  him ;  in  the  true  scien- 
tific spirit  he  replies,  "  Show  me  the  man  who  makes  no  mistakes,  and  I  will 
show  you  a  man  who  has  done  nothing." 


THE  CONNECTION  AND  EQUIVALENCE 
OF  FORCES. 


IT  is  well  known  that  our  machines  create  no  power,  but 
only  return  what  they  have  received.  The  motion  of  a 
clock  is  produced  by  a  weight  or  a  spring ;  but  it  is  the  power 
of  the  human  arm  applied  to  stretch  the  spring  or  elevate  the 
weight,  which  is  expended  in  the  movement  of  the  wheels  and 
pendulum  in  twenty-four  hours,  or  in  eight  or  fourteen  days. 

A  water-wheel  sets  in  motion,  in  a  mill,  one  or  more  mill- 
stones ;  in  a  foundry,  one  or  more  hammers ;  in  saltworks 
or  mines  it  pumps  or  raises  weights  to  certain  heights ;  in 
factories,  it  communicates  movements  to  looms,  spinning  ma- 
chines, and  rollers.  In  all  these  instances,  the  work  per- 
formed by  the  water-wheel  is  due  to  the  force  exerted  by  the 
falling  water  on  the  buckets,  which  sets  the  wheel  in  motion ; 
and  this  force  must  be  greater  than  the  resistance  presented 
by  the  different  machines  in  operation.  The  performance  of 
the  machine  is  measurable  by  this  force. 

The  work  of  a  steam-engine  is  executed  by  the  movement 
of  a  piston  upwards  and  downwards  by  the  pressure  of  steam, 
just  as  a  water-wheel  is  moved  by  the  pressure  of  water. 
The  cause  of  this  pressure  is  heat,  which  is  derived  from  the 
chemical  process  of  combustion,  and  is  absorbed  by  water. 
By  this  heat,  steam  is  produced,  and  the  necessary  expansion 


388      THE  CONNECTION  AND  EQUIVALENCE  OF  FOECES. 

obtained  for  the  movement  of  the  piston.  It  is  heat,  in  this 
last  form,  which  performs  the  mechanical  work  of  the  ma- 
chine. 

Every  force  acts  by  producing  pressure  either  from  or  tow- 
ards the  centre  of  motion.  In  every  machine  in  operation 
the  amount  of  power  is  always  measurable  by  the  resistance 
overcome  ;  and  this  again  can  be  expressed  by  corresponding 
weights,  which  that  power  is  capable  of  raising  to  a  certain 
height.  If  one  man  raises  by  a  pump,  in  one  minute,  150  Ibs. 
of  water,  and  another  200  Ibs. ;  or  if  one  horse  draws  to  a 
certain  distance  a  load  of  20  cwt.,  and  another  a  load  of  30 
cwt.,  it  is  evident  that  these  numbers  express  the  relative 
working  power  of  these  two  men  or  horses.  In  mechanics, 
the  working  power  of  every  machine  is  expressed  in  horse 
power,  that  is,  a  force  capable  of  elevating  in  each  second  75 
kilogrammes  (=2^  Ibs.  avoirdupois)  to  a  height  of  one  meter 
(39-37  inches). 

The  whole  power  communicated  to  a  machine  is  not  actu- 
ally available,  but  is  in  part  lost  by  friction.  For  if  two 
machines  possess  the  same  power,  it  is  found  that  the  greater 
quantity  of  work  will  be  executed  by  the  one  which  has  to 
overcome  the  smaller  amount  of  friction.  In  mechanics, 
friction  is  always  regarded  as  acting  in  direct  opposition  to 
motion  in  every  machine.  It  was  believed  that  the  working 
power  of  a  machine  could  be  absolutely  annihilated  by  it. 

As  the  proximate  cause  of  the  cessation  of  motion,  friction 
was  a  palpable  fact,  and  could  as  such  be  taken  into  account ; 
but  a  fatal  error  was  committed  in  giving  a  theoretical  view 
of  its  mode  of  action.  For  if  a  power  could  be  annihilated, 
or,  in  other  words,  have  nothing  as  its  effect,  then  there  would 
be  no  contradiction  involved  in  the  belief,  that  out  of  nothing 
also  power  could  be  created.  To  this  erroneous  idea  we  may 
partly  trace  the  belief,  held  for  centuries  by  most  able  men,  in 
the  possibility  x>f  discovering  a  machine  which  should  renew 
within  itself  its  own  power  as  it  was  expended,  and  thus  ever 


STATEMENT   OF   MAYEn's   ARGUMENT.  389 

continue  in  motion,  without  the  necessity  for  any  external 
motive  force.  The  discovery  of  such  a  perpetual  motion  was, 
indeed,  worthy  of  every  effort.  It  would  be  as  valuable  as 
the  bird  which  lays  the  golden  eggs  ;  for  by  its  means  labour 
would  be  performed,  and  money  made  in  abundance  without 
any  expenditure. 

A  mass  of  facts,  hitherto  unintelligible,  have  had  much 
light  thrown  upon  them  by  a  more  correct  view  of  natural 
forces,  for  which  we  are  indebted  to  a  physician,  Dr.  Mayer, 
of  Heilbronn,  and  which,  by  the  investigations  of  the  most 
eminent  natural  philosophers  and  mathematicians,  has  attained 
a  significance  and  importance  scarcely  to  be  foreseen. 

According  to  Dr.  Mayer,  forces  are  causes,  in  which  full 
application  of  the  axiom  must  be  found,  that  every  cause 
must  produce  an  effect  which  corresponds  and  is  equal  to  the 
cause.  Causa  cequat  effectum.  Thus  if  a  cause  C  produces 
an  effect  E,  then  C  =  E.  Should  the  effect  E  become  the 
cause  of  another  effect  e,  then  also  E  =  e  =  C.  In  such  a 
chain  of  causes  and  effects  no  link,  or  part  of  a  link,  can  ever 
become  nothing  =  nothing.  Should  a  given  cause  C  have  pro 
duced  its  corresponding  effect  E,  then  C  ceases  to  exist,  for 
it  has  been  converted  into  E.  Consequently,  as  C  passes  into 
E,  and  the  latter  into  e,  it  follows  that  all  these  causes,  as  far 
as  relates  to  their  quantity,  possess  the  property  of  indestructir 
bility,  and  to  their  quality  that  of  convertibility.  In  number- 
less cases  we  see  a  motion  cease,  without  its  usual  effects 
being  produced,  such  as  lifting  a  weight  or  load ;  but  as  the 
force  which  has  caused  the  motion  cannot  be  reduced  to 
nothing,  the  question  arises,  what  form  has  it  assumed.  Ex- 
perience gives  the  answer,  by  showing  that  wherever  motion 
is  arrested  by  friction,  a  blow,  or  concussion,  heat  is  the  re- 
sult. The  motion  is  the  cause  of  the  heat. 

The  rapid  friction  of  two  plates  of  metal  can  raise  their 
temperature  to  redness,  and  cause  the  ebullition  of  water  if 
the  friction  takes  place  below  its  surface.  In  like  manner. 


390      THE  CONNECTION  AND  EQUIVALENCE  OF  FORCES. 

by  rapid  motion  tne  iron  tires  of  carriage  wheels  become  fre- 
quently so  hot  that  they  cannot  be  touched.  In  grinding 
needle-points  the  steel  is  heated  to  redness,  and  the  detached 
particles  burn  with  sparks.  The  wooden  breaks  of  railway 
carriages  become  frequently  so  hot  by  friction  that  their  sur- 
face emits  an  empyreumatic  odour.  'By  the  friction  of  an 
iron  grater,  particles  of  white  sugar  can  be  melted  and  heated 
so  far  as  to  acquire  the  taste  of  burnt  sugar  (Caramel) .  The 
heat  evolved  by  the  friction  of  two  pieces  of  ice  is  sufficient  to 
melt  them. 

In  the  English  steel-foundries  a  bar  of  steel  10  or  12 
inches  long,  by  being  heated  at  one  end  in  a  forge,  is  welded 
by  hammering  to  another  slender  bar,  10  or  12  feet  long, 
without  the  necessity  of  further  direct  application  of  heat — a 
point  of  great  importance  for  the  preservation  of  the  good 
quality  of  the  steel.  Every  spot  on  which  the  powerful  blows 
of  the  hammer  rapidly  descend,  becomes  red  hot,  and  to  the 
spectators  the  red  glow  of  heat  appears  to  run  up  and  down 
the  bar.  This  glow  is  produced  by  the  blows  of  the  hammer, 
and  corresponds  to  an  amount  of  heat  sufficient  to  raise  many 
pounds  of  water  to  the  boiling  point ;  whilst  the  end  of  the 
bar  heated  in  the  fire  would  scarcely  by  itself  raise  to  the 
same  temperature  as  many  ounces  of  water. 

According  to  the  preceding  views  a  precise  connection 
exists  between  the  blows  of  the  hammer  (the  cause)  and  the 
heat  produced  (the  effect)  ;  and  natural  philosophers  have 
devised  the  most  ingenious  experiments  to  show  this  relation. 
The  working  power  is  in  this  case  converted  into  heat.  If 
the  view  of  Mayer  be  correct,  then  should  we  by  this  amount 
of  heat  thus  obtained,  be  able  to  reproduce  the  same  amount 
of  work,  viz.,  the  same  number  of  blows  of  the  hammer.  But 
a  closer  view  of  the  point  shows  that  we  require  to  elevate 
the  hammer,  and  that  therefore  its  working  power  was  not 
inherent,  but  only  lent  to  it.  The  hammer  was  elevated  by  a 
water-wheel  set  in  motion  by  a  certain  weight  of  water  falling 


MECHANICAL   POWEE   CHANGED   TO   HEAT.  391 

on  its  buckets.  Thus  to  raise  a  hammer  wefghing  ten  pounds 
to  the  height  of  one  foot  requires  at  least  the  fall  of  ten  pounds 
weight  of  water  from  a  height  of  a  foot.  It  was  then,  properly 
speaking,  this  weight  of  falling  water  which  produced  the 
heat  through  means  of  the  hammer.  By  simply  altering  the 
arrangement  of  the  machinery,  the  same  force  would  have 
caused  a  mill-stone  to  revolve  with  great  rapidity  on  its  axis, 
or  raised  by  friction  two  iron  disks  to  a  red  heat. 

From  experiments  instituted  to  elucidate  this  point,  it  has 
been  established  that  13,500  blows  of  a  hammer,  weighing  10 
pounds,  falling  on  a  bar  of  iron  from  a  height  of  one  foot,  pro- 
duce an  amount  of  heat  sufficient  to  raise  one  pound  of  water 
from  the  freezing  point  to  that  of  ebullition.  This  fact  may 
be  represented  in  another  way  by  saying,  that  1,350  cwts.  of 
water,  falling  from  a  height  of  one  foot,  will  raise  the  temper- 
ature of  1  Ib.  of  water  from  freezing  to  the  boiling  point ;  or 
1,350  Ibs.  of  water  falling  from  the  same  height  will  raise  one 
pound  of  water  one  degree  in  temperature,  or  in  other  words, 
that  this  amount  of  heat  corresponds  to  a  working  power, 
capable  of  elevating  13^  cwt.  to  the  height  of  one  foot. 

Wherever  motion  is  lost  in  a  machine  by  friction  or  by 
concussion,  there  is  always  produced  a  corresponding  amount 
of  heat.  When,  on  the  other  hand,  a  certain  quantity  of 
work  is  performed  by  heat,  there  disappears,  with  the  me- 
chanical effect  obtained,  a  certain  amount  of  heat,  which  is 
expressed  by  saying  that  the  heat  lost  by  one  pound  of  water 
in  falling  one  degree  in  temperature,  is  equal  to  the  elevation 
of  13 J  cwt.  to  the  height  of  one  foot.  This  quantity  of  heat 
becomes  then  the  equivalent  or  value  of  the  working  power 
expressed  by  the  above  numbers. 

This  constant  relation  between  heat  and  mechanical  move- 
ment has  been  confirmed  in  the  most  varied  manner.  A  rod 
of  metal  is  extended  by  a  weight,  and  on  its  removal  resumes 
its  original  length,  provided  certain  limits  be  not  exceeded. 
The  same  effect  is  produced  by  heat ;  and  it  is  evident  that 


.892      THE  CONNECTION  AND  EQUIVALENCE  OF  FOECE8. 

an  equal  force  must  be  exerted  by  the  rod  in  its  extension 
as  in  its  contraction.  Now,  experiment  has  shown  that  the 
relation  expressed  by  the  numbers  above  given,  must  exist 
between  a  given  extension  of  a  bar  of  iron,  and  the  heat  or 
weight  which  has  caused  that  extension,  viz.,  that  a  quantity 
of  heat  sufficient  to  raise  a  pound  of  water  one  degree  in 
temperature,  will,  when  communicated  to  a  bar  of  iron,  en- 
able it  to  elevate  a  weight  of  1,350  Ibs.  to  the  height  of  one 
foot. 

An  interesting  application  of  this  fact  was  long  ago  made 
in  the  Conservatoire  des  Arts  et  Metiers,  in  Paris.  In  this 
building,  which  was  formerly  a  convent,  the  nave  of  the  church 
was  converted  into  a  museum  for  industrial  products,  machines, 
and  implements.  In  its  arch,  traversing  its  length,  appeared 
a  crack,  which  gradually  increased  to  the  width  of  several 
inches,  and  permitted  the  passage  of  rain  and  snow.  The 
opening  could  easily  have  been  closed  by  stone  and  lime,  but 
the  yielding  of  the  side  walls  would  not  have  been  prevented 
by  these  means.  The  whole  building  was  on  the  point  of 
being  pulled  down,  when  a  natural  philosopher  proposed  the 
following  plan,  by  which  the  object  was  accomplished.  A 
number  of  strong  iron  rods  were  firmly  fixed  at  one  end  to  a 
.side  wall  of  the  nave,  and  after  passing  through  the  opposite 
wall  were  provided  on  the  outside  with  large  nuts,  which 
were  screwed  up  tightly  to  the  wall.  By  applying  burning 
straw  to  the  rods,  they  extended  in  length.  The  nuts  by  this 
extension  being  now  removed  several  inches  from  the  wall, 
were  again  screwed  tight  to  it.  The  rods  on  cooling  con- 
tracted with  enormous  force,  and  made  the  side  walls  ap- 
proach each  other.  By  repeating  the  operation  the  crack 
entirely  disappeared.  This  building  with  its  retaining  rods 
is  still  in  existence. 

The  working  power  of  a  machine,  set  in  motion  by  elec- 
tricity, can  be  expressed  by  numbers,  in  the  same  way  as  the 
mechanical  effect  of  heat.  An  electrical  current  is  generated 


ELECTEICITY   CONVERTED   INTO   HEAT.  393 

by  a  rotating  magnet  or  by  solution  of  zinc  in  the  galvanic 
battery.  Such  a  current,  in  circulating  through  a  thick  or 
thin  wire,  exhibits  the  same  deportment  as  a  fluid  flowing 
through  a  wide  or  narrow  tube.  As  a  given  quantity  of  fluid 
requires  more  time  or  greater  pressure  to  pass  through  a  nar- 
row tube  than  through  a  large,  so  a  thin  wire  offers  a  greater 
resistance  than  a  thick  one  to  the  passage  of  a  current  of  elec- 
tricity. The  current  is  thus  retarded  and  diminished,  one 
portion  only  passing  through  the  conductor,  the  other  being 
converted  into  heat.  According  to  the  amount  of  heat  thus 
produced  by  the  conversion  of  the  electricity,  a  conducting 
wire  of  platinum  can  be  fused,  one  of  gold  fused  and  con- 
verted into  vapour,  and  a  considerable  quantity  of  water 
brought  into  violent  ebullition  by  passing  the  current  through 
a  thin  platinum  wire  wound  round  a  glass  tube  in  a  spiral 
form. 

If  the  electrical  current  circulates  through  a  wire  wound 
spirally  round  a  bar  of  iron,  the  latter  is  converted  into  a 
powerful  magnet  capable  of  attracting  and  carrying  several 
hundred  weights  of  iron.  The  electrical  is  converted  into  the 
magnetic  force,  by  which  a  machine  may  be  set  in  motion. 
The  power  of  attraction  communicated  to  the  iron  bar  is  in 
exact  proportion  to  the  amount  of  electricity  circulating  in 
the  surrounding  wire,  and  this  current  is  again  dependent  on 
the  property  of  the  conductor.  That  portion  of  electricity 
which  in  the  conductor  is  converted  into  heat,  produces  no 
power  of  attraction  in  the  iron  bar.  It  follows,  from  the 
foregoing,  that  the  quantity  of  electricity  which  circulates,  of 
that  which  produces  heat,  and  the  amount  of  magnetic  power 
convertible  into  working  power,  stand  in  the  same  relation  to 
each  other,  as  the  working  power  produced  in  a  machine  by 
the  pressure  of  falling  water  to  the  heat  generated  by  friction 
and  concussion  in  the  same  machine.  The  same  amount  of 
electricity  which,  when  converted  into  heat  by  the  resistance 
of  the  conductor,  raises  by  one  degree  the  temperature  of  one 


394      THE  CONNECTION  AND  EQUIVALENCE  OF  FORCES. 

pound  of  water,  generates  a  magnetic  force  capable  of  ele- 
vating a  weight  of  13-J-  cwt.  to  the  height  of  one  foot. 

If  the  metallic  wire  through  which  the  electricity  is  cir- 
culating, be  cut,  and  both  ends  immersed  in  water,  a  chemical 
decomposition  of  the  water  into  hydrogen  and  oxygen  takes 
place.  The  circulating  electricity  is  converted  into  chemical 
affinity,  and  into  a  power  of  attraction  which  causes  the  sep- 
aration of  the  elements  of  water.  With  the  evolution  of  the 
hydrogen  and  oxygen  all  traces  of  the  electrical  current  dis- 
appear. The  power  to  produce  heat  and  magnetic  force,  the 
usual  effects  of  the  electrical  current,  is  apparently  in  this 
case  annihilated,  and  in  its  place  we  obtain  two  gases,  one  of 
which,  hydrogen,  when  burned  in  oxygen,  reproduces  water 
and  evolves  heat.  Now  it  has  been  proved,  by  careful  ex- 
periments, that  an  electrical  current  of  a  given  strength, 
which,  when  converted  into  heat  in  a  conductor,  is  capable 
of  raising  the  temperature  of  a  pound  of  water  by  one  degree, 
will  produce  by  the  decomposition  of  water  a  quantity  of  hy- 
drogen, by  the  combustion  of  which  one  pound  of  water  can 
also  be  elevated  one  degree  in  temperature. f 

The  heat  and  power  of  attraction  which  were  apparently 
lost  by  the  decomposition  of  the  water,  had  only  become 
latent,  so  to  speak,  in  the  elements  of  water.  This  heat  is 
again  set  free  on  the  reunion  of  these  elements,  and  if  con- 
verted into  working  power,  would  produce  the  same  result 
(viz.,  raising  a  given  weight  a  foot  high)  as  would  have  been 
effected  by  a  magnetic  power  generated  by  a  quantity  of  elec- 
tricity circulating  round  a  bar  of  iron,  equal  to  that  which 
was  originally  employed  in  the  decomposition  of  the  water. 

The  electrical  current  is  the  consequence  of  a  chemical 
action,  and  the  amount  of  electricity  which  circulates  can 
therefore  be  measured  by  the  quantity  of  zinc  which  is  dis- 
solved. The  chemical  force  (affinity)  is  converted  by  the 
solution  of  the  zinc  into  a  corresponding  quantity  of  elec- 
tricity ;  and  this  again  in  the  conductor  into  its  equivalent  of 


THE  MOTIVE-FOKCE  OF   PLANTS.  395 

heat,  or  into  magnetic  force,  or,  as  in  the  case  of  the  decom- 
position of  water,  into  chemical  force.  In  no  case  is  there  a 
diminution  or  increase  of  force.  If,  according  to  the  materi- 
alist, matter  is  indestructible,  the  same  holds  good  with  regard 
to  force.  It  is  not  extinguished ;  its  apparent  annihilation, 
its  disappearance,  is  only  a  conversion  into  some  other  form. 

We  know  now  the  origin  of  the  heat  and  light  which 
warm  and  illuminate  our  dwellings,  of  the  heat  and  power 
generated  in  our  bodies  by  the  vital  process.  Plants  are  the 
one  source  of  all  materials  used  for  the  production  of  heat 
and  light,  and  of  that  nourishment  which  must  be  daily  taken 
to  maintain  the  phenomena  of  vitality.  The  elements  of 
plants  are  earthy  in  their  nature,  and  are  obtained  from 
water,  earth,  and  air.  In  plants,  certain  inorganic  com- 
pounds— carbonic  acid,  water,  and  ammonia-r-are  decom- 
posed. The  carbon  of  the  carbonic  acid,  the  hydrogen  of 
the  water,  and  the  nitrogen  of  the  ammonia,  are  retained  as 
constituents  of  their  organs,  but  the  oxygen  of  the  carbonic 
acid  and  of  the  water  are  returned  as  gas  to  the  air.  With 
out  light,  however,  plants  cannot  grow. 

The  vital  process  in  plants  exhibits  itself  as  directly  oppo- 
site in  its  character  to  the  chemical  process  in  the  formation 
of  salts.  f 

Carbonic  acid,  water,  and  zinc,  when  brought  together  pro- 
duce a  certain  effect  on  each  other.  In  virtue  of  chemical 
affinity  there  is  formed  a  white  powdery  compound,  contain- 
ing carbonic  acid,  zinc,  and  oxygen  from  the  water,  and 
hydrogen  is  at  the  same  time  evolved. 

In  plants,  the  living  bud  or  part  of  the  plant  takes  the 
place  of  the  zinc.  By  their  growth  are  formed,  from  carbonic 
acid  and  water,  compounds  containing  carbon  and  hydrogen, 
or  carbon  and  the  elements  of  water,  and  oxygen  is  at  the 
same  time  evolved.  Sunlight  acts  in  living  plants  like  elec- 
tricity, which  arrests  the  natural  attraction  of  the  elements  of 
water,  and  separates  them  from  each  other. 


396      THE  COHNECTIOK  AND  EQUIVALENCE  OF  FORCES* 

"Without  the  light  of  the  sun  plants  cannot  grow.  The 
living  germ,  the  green  leaf,  owe  to  the  sun  their  power  of 
transforming  earthy  elements  into  living,  vigorous  structures. 
The  germ  may,  indeed,  be  evolved  under  ground  without  the 
action  of  light,  but  only  when  it  breaks  through  the  surface 
of  the  soil  does  it  first  acquire  the  power,  by  the  sun's  rays, 
of  converting  inorganic  elements  into  its  own  structure.  The 
illuminating  and  heating  rays  of  the  sun,  in  thus  bestowing 
life,  lose  their  own  light  and  heat.  Their  power  now  becomes 
latent  in  the  new  products  of  the  frame,  which  have  been 
produced  under  their  influence  from  carbonic  acid,  water,  and 
ammonia.  The  light  and  heat  with  which  our  dwellings  are 
illuminated  and  warmed  are  but  those  bestowed  by  the  sun. 

The  food  of  men  and  animals  consists  of  two  classes  of 
materials,  which  differ  totally  in  their  nature.  One  class  is 
destined  to  the  production  of  blood  and  the  maintenance  of 
the  structure  of  the  body.;  the  other  is  similar  in  composition 
to  ordinary  materials  for  combustion.  Sugar,  starch,  the 
gum  of  bread,  may  be  regarded  as  transformed  woody  fibre, 
for  we  can  prepare  them  from  this  substance.  Fat,  in  its 
amount  of  carbon,  resembles  closely  mineral  coal.  We  heat 
our  bodies  as  we  do  stoves,  by  combustibles  which  possess  the 
same  elements  as  wood  and  coal,  but  which  differ  essentially 
from  them  by  being  soluble  in  the  fluids  of  the  body. 

The  elements  of  nutrition  from  which  the  temperature  of 
the  body  is  derived,  evidently  produce  no  mechanical  power ; 
because  power  is  but  converted  heat,  and  the  heat  which 
maintains  and  elevates  the  temperature  of  the  body  does  not 
produce  any  other  effect  than  that  of  warmth. 

All  those  mechanical  operations  constantly  taking  place  in 
the  living  body,  in  the  movement  of  organs  and  limbs,  are 
dependent  on  an  accompanying  change  in  the  composition 
and  properties  of  those  highly  complex  sulphur  and  nitrogen 
constituents  of  the  muscles,  which,  though  furnished  by  the 
blood,  are  in  the  first  instance  derived  directly  from  the  food 


DERIVATION   OF  ANIMAL   POWER.  397 

of  man.  The  change  of  position  in  the  elements  of  these 
complex  bodies,  attendant  on  their  rearrangement  into  new 
and  simpler  compounds,  necessarily  gives  rise  to  motion  ;  and 
the  molecular  movement  of  the  particles  in  a  state  of  change 
is  transferred  to  the  muscular  mass.  Chemical  action  is  thus 
evidently  the  source  of  mechanical  power  in  bodies. 

The  elements  of  the  food  of  men  and  animals  which  give 
rise  to  power  and  heat,  are  produced  in  living  plants  only  by 
the  action  of  sunlight.  The  rays  of  the  sun  become  latent, 
so  to  speak,  in  them  in  the  same  way  as  the  current  of  elec- 
tricity becomes  latent  in  the  hydrogen  by  decomposition  of 
water. 

Man,  by  food,  not  only  maintains  the  perfect  structure  of 
his  body,  but  he  daily  lays  in  a  store  of  power  and  heat,  de- 
rived in  the  first  instance  from  the  sun.  This  power  and 
heat,  latent  for  a  time,  reappears  and  again  becomes  active 
when  the  living  structures  are  resolved  by  the  vital  processes 
into  their  original  elements. 

The  rays  of  the  sun  add  daily  to  the  store  of  indestructi- 
ble forces  of  our  terrestrial  body,  maintaining  life  and  motion. 
Thus,  from  beyond  the  limits  of  our  earth,  the  body,  the  more 
earthly  vessel,  derives  all  that  may  be  called  good  in  it,  and 
of  this  not  a  single  particle  is  ever  lost. 


THE  COEKELATION 

OF   THE 

PHYSICAL   AND   VITAL   FOKCES. 

BY  DR.  WILLIAM  B.  CARPENTER. 


WILLIAM  BENJAMIN  CARPENTER,  the  eminent  English  physiologist,  was 
born  in  the  early  part  of  this  century,  and  graduated  as  doctor  of  medicine 
in  Edinburgh  in  1839.  He  commenced  practice  in  Bristol,  but  has  been 
chiefly  occupied  as  a  lecturer  and  author.  His  most  important  works  are 
the  "Principles  of  General  and  Comparative  Physiology"  (1839),  the 
"  Principles  of  Human  Physiology"  (1846),  which  reached  a  fifth  edition  in 
1855,  and  "  The  Microscope,  its  Revelations  and  Uses"  (1856).  He  is  the 
author  of  various  minor,  but  valuable  works,  and  of  many  elaborate  papers 
in  the  cyclopedias  and  scientific  periodicals.  For  many  years  he  was  editor 
of  the  "  British  and  Foreign  Medico-Chirurgical  Review."  He  was  elected 
a  member  of  the  Royal  Society  in  1844,  and  is  now  Professor  of  Medical 
Jurisprudence  in  University  College,  London ;  Lecturer  on  General  Anatomy 
and  Physiology  at  the  London  Hospital  and  School  of  Medicine,  and  Ex- 
aminer in  Physiology  and  Comparative  Anatomy  in  the  University  of  Lon- 
don. In  1850  he  published  an  able  paper  hi  the  Transactions  of  the  Royal 
Society  on  the  "  Mutual  Relations  of  the  Vital  and  Physical  Forces,"  and 
has  published  upon  the  same  general  subject,  the  essays  which  close  this  vol- 
ume, in  the  new  Quarterly  Journal  of  Science  for  the  present  year.  Dr.  Car- 
penter is  an  able  and  original  thinker,  as  well  as  a  voluminous  writer,  and  has 
made  many  valuable  contributions  to  the  progress  of  physiological  science. 


ON  THE  COKKELATION  OF  THE  PHYS- 
ICAL AND  VITAL  FOKCES. 


I.— RELATIONS  OF  LIGHT  AND  HEAT  TO  THE  VITAL  FORCES 
OF  PLANTS. 


IN  every  period  of  the  history  of  Physiology,  attempts 
have  been  made  to  identify  all  the  forces  acting  in  the 
Living  Body  with  those  operating  in  the  Inorganic  universe. 
Because  muscular  force,  when  brought  to  bear  on  the  bones, 
moves  them  according  to  the  mechanical  laws  of  lever  action, 
and  because  the  propulsive  power  of  the  heart  drives  the 
blood  through  the  vessels  according  to  the  rules  of  hydraulics, 
it  has  been  imagined  that  the  movements  of  living  bodies  may 
be  explained  on  physical  principles  ;  the  most  important  con- 
sideration of  all,  namely,  the  source  of  that  contractile  power 
which  the  living  muscle  possesses,  but  which  the  dead  muscle 
,  (though  having  the  same  chemical  composition)  is  utterly  in- 
capable of  exerting,  being  altogether  left  out  of  view.  So, 
again,  because  the  digestive  process,  whereby  food  is  reduced 
to  a  fit  state  for  absorption,  as  well  as  the  formation  of  va- 
rious products  of  the  decomposition  that  is  continually  taking 
place  in  the  living  body,  may  be  initiated  in  the  laboratory 
of  the  chemist ;  it  has  been  supposed  that  the  appropriation 


402       COKKELATION   OF   PHYSICAL   AND   VITAL   FOKCES. 

of  the  nutriment  to  the  production  of  the  living  organized 
tissues  of  which  the  several  parts  of  the  body  are  composed, 
is  to  be  regarded  as  a  chemical  action — as  if  any  combination 
of  albumen  and  gelatine,  fat  and  starch,  salt  and  bone-earth, 
could  make  a  living  Man  without  the  constructive  agency  in- 
herent in  the  germ  from  which  his  bodily  fabric  is  evolved. 

Another  class  of  reasoners  have  cut  the  knot  which  they 
could  not  untie,  by  attributing  all  the  actions  of  living  bodies 
for  which  Physics  and  Chemistry  cannot  account,  to  a  hypo- 
thetical u  Vital  principle  ;"  a  shadowy  agency  that  does  every 
thing  in  its  own  way,  but  refuses  to  be  made  the  subject  of 
scientific  examination ;  like  the  "  od-force"  or  the  "  spiritual 
power  "  to  which  the  lovers  of  the  marvellous  are  so  fond  of 
attributing  the  mysterious  movements  of  turning  and  tilting 
tables. 

A  more  scientific  spirit,  however,  prevails  among  the  best 
Physiologists  of  the  present  day ;  who,  whilst  fully  recogniz- 
ing the  fact  that  many  of  the  phenomena  of  living  bodies  can 
be  accounted  for  by  the  agencies  whose  operation  they  trace 
in  the  world  around,  separate  into  a  distinct  category — that 
of  vital  actions — such  as  appear  to  differ  altogether  in  kind 
from  the  phenomena  of  Physics  and  Chemistry,  and  seek  to 
determine,  from  the  study  of  the  conditions  under  which  these 
present  themselves,  the  laws  of  their  occurrence. 

In  the  prosecution  of  this  inquiry,  the  Physiologist  will 
find  it  greatly  to  his  advantage  to  adopt  the  method  of  philos- 
ophizing which  distinguishes  the  Physical  science  of  the  pres- 
ent from  that  of  the  past  generation ;  that,  namely,  which, 
whilst  fully  accepting  the  logical  definition  of  the  cause  of  any 
phenomenon,  as  "  the  antecedent,  or  the  concurrence  of  ante- 
cedents on  which  it  is  invariably  and  unconditionally  conse- 
quent" (Mill),  draws  a  distinction  between  the  dynamical 
and  the  material  conditions  ;  the  former  supplying  the  power 
which  does  the  work,  whilst  the  latter  affords  the  instrumental 
means  through  which  that  power  operates. 


DYNAMICAL   AND  MATEEIAL   CONDITIONS.  403 

Thus  if  we  inspect  a  Cotton  Factory  in  full  action,  we  find 
it  to  contain  a  vast  number  of  machines,  many  of  them  but 
repetitions  of  one  another,  but  many,  too,  presenting  the  most 
marked  diversities  in  construction,  in  operation,  and  in  result- 
ant products.  We  see,  for  example,  that  one  is  supplied  with 
the  raw  material,  which  it  cleans  and  dresses ;  that  another 
receives  the  cotton  thus  prepared,  and  "  cards  "  it  so  as  to  lay 
its  fibres  in  such  an  arrangement  as  may  admit  of  its  being 
spun  ;  that  another  series,  taking  up  the  product  supplied  by 
the  carding  machine,  twists  and  draws  it  out  into  threads  of 
various  degrees  of  fineness  ;  and  that  this  thread,  carried  into 
a  fourth  set  of  machines,  is  woven  into  a  fabric  which  may 
be  either  plain  or  variously  figured  according  to  the  construc- 
tion of  the  loom.  In  every  one  of  these  dissimilar  operations 
the  force  which  is  immediately  concerned  in  bringing  about  the 
results  is  one  and  the  same  ;  and  the  variety  of  its  products  is 
dependent  solely  upon  the  diversity  of  the  material  instru- 
ments through  which  it  operates.  Yet  these  arrangements, 
however  skilfully  devised,  are  utterly  valueless  without  the 
force  which  brings  them  into  play.  All  the  elaborate  mechan- 
ism, the  triumph  of  human  ingenuity  in  devising,  and  of  skill 
in  constructing,  is  as  powerless  as  a  corpse  without  the  vis 
viva  which  alone  can  animate  it.  The  giant  stroke  of  the 
steam-engine,  or  the  majestic  revolution  of  the  water-wheel 
gives  the  required  impulse ;  and  the  vast  apparatus  which 
was  the  moment  previously  in  a  state  of  death-like  inactivity, 
is  aroused  to  all  the  energy  of  its  wondrous  life — every  part 
of  its  complex  organization  taking  upon  itself  its  peculiar 
mode  of  activity,  and  evolving  its  own  special  product,  in  vir- 
tue of  the  share  it  receives  of  the  one  general  force  distrib- 
uted through  the  entire  aggregate  of  machinery. 

But  if  we  carry  back  our  investigation  a  stage  further,  and 
inquire  into  the  origin  of  the  force  supplied  by  the  steam- 
engine  or  the  water-wheel,  we  soon  meet  with  a  new  and 
most  significant  fact.  At  our  first  stage,  it  is  true,  we  find 


404      CORRELATION   OF   PHYSICAL   AND   VITAL   FORCES. 

only  the  same  mechanical  force  acting  through  a  different 
kind  of  instrumentality ;  the  strokes  of  the  piston  of  the 
steam-engine  being  dependent  upon  the  elastic  force  of  the 
vapour  of  water,  whilst  the  revolution  of  the  water-wheel  is 
maintained  by  the  downward  impetus  of  water  en  masse. 
But  to  what  antecedent  dynamical  agency  can  we  trace  these 
forces  ?  That  agency  in  each  case  is  Heat ;  a  force  altogether 
dissimilar  in  its  ordinary  manifestations  to  the  force  which 
produces  sensible  motion,  yet  capable  of  being  in  turn  con- 
verted into  it  and  generated  by  it.  For  it  is  from  the  Heat 
applied  beneath  the  boiler  of  the  steam-engine  that  the  non- 
elastic  liquid  contained  in  it  derives  all  that  potency  as  elastic 
vapour,  which  enables  it  to  overcome  the  vast  mechanical  re- 
sistance that  is  set  in  opposition  to  it.  And,  in  like  manner, 
it  is  the  heat  of  the  solar  rays  which  pumps  up  terrestrial 
waters  in  the  shape  of  vapour,  and  thus  supplies  to  Man  a 
perrenial  source  of  new  power  in  their  descent  by  the  force 
of  gravity  to  the  level  from  which  they  have  been  raised.* 

The  power  of  the  steam-engine,  indeed,  is  itself  derived 
more  remotely  from  those  same  rays ;  for  the  Heat  applied 
to  its  boilers  is  but  the  expression  of  the  chemical  change  in- 
volved in  combustion  ;  that  combustion  is  sustained  either  by 
the  wood  which  is  the  product  of  the  vegetative  activity  of 
the  present  day,  or  by  the  coal  which  represents  the  vegeta- 
tive life  of  a  remote  geological  epoch ;  and  that  vegetative 
activity,  whether  present  or  past,  represents  an  equivalent 
amount  of  Solar  Light  and  Heat,  used  up  in  the  decomposi- 
tion of  the  carbonic  acid  of  the  atmosphere,  by  the  instru- 
mentality of  the  growing  plant.f  Thus  in  either  case  we 
come,  directly  or  indirectly,  to  Solar  Radiation  as  the  main- 

*  See  on  this  subject  the  recent  admirable  address  of  Sir  William  Arm- 
strong at  the  Meeting  of  the  British  Association  at  Newcastle. 

f  This  was  discerned  by  the  genius  of  George  Stephenson,  before  the 
general  doctrine  of  the  Correlation  of  Forces  had  been  given  to  the  world 
by  Mayer  and  Grove. 


ESTABLISHMENT    OF  THE   GENERAL   PRINCIPLE.      405 

spring  of  our  mechanical  power ;  the  vis  viva  of  our  whole 
microcosm.  Modern  physical  inquiry  ventures  even  one  step 
further,  and  seeks  the  source  of  Light  and  Heat  of  the  Sun 
itself.  Are  these,  as  formerly  supposed,  the  result  of  com- 
bustion, or  are  they,  as  surmised  by  Mayer  and  Thomson, 
the  expression  of  the  motive  power  continually  generated  in 
the  fall  of  aerolites  towards  the  Sun,  and  as  continually  anni- 
hilated by  their  impact  on  its  surface  ?  Leaving  the  discus- 
sion of  this  question  to  Physical  Philosophers,  I  proceed  now 
to  my  own  proper  subject. 

It  is  now  about  twenty  years  since  Dr.  Mayer  first  broadly 
announced,  in  all  its  generality,  the  great  principle  now  known 
as  that  of  "  Conservation  of  Force ;"  as  a  necessary  deduc- 
tion from  two  axioms  or  essential  truths — ex  nihilo  nil  fit,  and 
nil  fit  ad  nihilum — the  validity  of  which  no  true  philosopher 
would  ever  have  theoretically  questioned,  but  of  which  he 
was  (in  my  judgment)  the  first  to  appreciate  the  full  practical 
bearing.  Thanks  to  the  labours  of  Faraday,  Grove,  Joule, 
Thomson,  and  Tyndall,  to  say  nothing  of  those  of  Helmholtz 
and  other  distinguished  Continental  savans,  the  great  doctrine 
expressed  by  the  term  "  Conservation  of  Force "  is  now 
amongst  the  best-established  generalizations  of  Physical  Sci- 
ence ;  and  every  thoughtful  Physiologist  must  desire  to  see 
the  same  course  of  inquiry  thoroughly  pursued  in  regard  to 
the  phenomena  of  living  bodies.  This  ground  was  first  broken 
by  Dr.  Mayer  in  his  remarkable  treatise,  *'  Die  Organische 
Bewegung  in  ihrem  Zusammenhange  nit  dem  Sloffwechsel " 
("  On  Organic  Movement  in  its  relation  to  Material  Changes," 
Heilbronn,  1845)  ;  in  which  he  distinctly  set  forth  the  princi- 
ple that  the  source  of  all  changes  in  the  living  Organism, 
animal  as  well  as  vegetable,  lies  in  the  forces  acting  upon  it 
from  without;  whilst  the  changes  in  its  own  composition 
brought  about  by  these  agencies,  he  considers  to  be  the  imme- 
diate source  of  the  forces  which  are  generated  by  it. 

In  treating  of  these  forces,  however,  he  dwells  chiefly  on 


4:06      COEEELATION  OF  PHYSICAL  AND  VITAL  FORCES. 

the  production  of  Motion,  Heat,  Light,  and  Electricity  by 
living  bodies  ;  touching  more  slightly  upon  the  phenomena  of 
Growth  and  Development,  which  constitute,  in  the  eye  of  the 
physiologist,  the  .distinct  province  of  vitality.  In  a  memoir 
of  my  own,  "  On  the  Mutual  Relations  of  the  Vital  and  Phys- 
ical Forces,"  published  in  the  Philosophical  Transactions  for 
1850,*  I  aimed  to  show  that  the  general  doctrine  of  the  "  Cor- 
relation of  the  Physical  Forces  "  propounded  by  Mr.  Grove, 
was  equally  applicable  to  those  Vital  forces  which  must  be 
assumed  as  the  moving  powers  in  the  production  of  purely 
physiological  phenomena ;  these  forces  being  generated  in 
living  bodies  by  the  transformation  of  the  Light,  Heat,  and 
Chemical  Action  supplied  by  the  world  around,  and  being 
given  back  to  it  again,  either  during  their  life,  or  after  its 
cessation,  chiefly  in  Motion  and  Heat,  but  also  to  a  less  de- 
gree in  Light  and  Electricity.  This  memoir  attracted  but 
little  attention  at  the  time,  being  regarded,  I  believe,  as  too 
speculative  ;  but  I  have  since  had  abundant  evidence  that  the 
minds  of  thoughtful  Physiologists,  as  well  as  Physicists,  are 
moving  in  the  same  direction  ;  and  as  the  progress  of  science 
since  the  publication  of  my  former  memoir,  would  lead  me 
to  present  some  parts  of  my  scheme  of  doctrine  in  a  different 
form,f  I  venture  to  bring  it  again  before  the  public  in  the 
form  of  a  sketch  (I  claim  for  it  no  other  title),  of  the  aspect 
in  which  the  application  of  the  principle  of  the  "  Conserva- 
tion of  Force  "  to  Physiology  now  presents  itself  to  my  mind. 

*  At  this  date  the  labours  of  Dr.  Mayer  were  not  known  either  to  my- 
self or  (so  far  as  I  am  aware)  to  any  one  else  in  this  country,  save  the  late 
Dr.  Baly,  who  a  few  months  after  the  publication  of  my  Memoir,  placed  in 
my  hands  the  pamphlet,  "  Die  Organische  Bewegung ; "  to  which  I  took  the 
earliest  opportunity  in  my  power  of  drawing  public  attention  in  "  The  Brit- 
ish and  Foreign  Medico-Chirurgical  Review  "  for  July,  1851,  p.  237. 

f  I  have  especially  profited  by  a  Memoir  on  the  Correlation  of  Physical, 
Chemical,  and  Vital  Force,  and  the  Conservation  of  Force  in  Vital  Phenom- 
ena, by  Prof.  Le  Conte  (of  South  Carolina  College),  in  Silliman's  American 
Journal  for  Nov.,  1859,  reprinted  in  the  Philosophical  Magazine  for  1860. 


CHAEACTERICnCS  OF   VITAL   ACTIVITY.  407 

If,  in  the  first  place,  we  inquire  what  it  is  that  essentially 
distinguishes  Vital  from  every  kind  of  Physical  activity,  we 
find  this  distinction  most  characteristically  expressed  in  the 
fact  that  a  germ  endowed  with  Life,  developes  itself  into  an 
organism  of  a  type  resembling  that  of  its  parent ;  that  this 
organism  is  the  subject  of  incessant  changes,  which  all  tend 
in  the  first  place  to  the  evolution  of  its  typical  form,  and  sub- 
sequently to  its  maintenance  in  that  form,  notwithstanding  the 
antagonism  of  Chemical  and  Physical  agencies,  which  are 
continually  tending  to  produce  its  disintegration ;  but  that,  as 
its  term  of  existence  is  prolonged,  its  conservative  power  de- 
clines so  as  to  become  less  and  less  able  to  resist  these  disinte- 
grating forces,  to  which  it  finally  succumbs,  leaving  the  organ- 
ism to  be  resolved  by  their  agency  into  the  components  from 
which  its  materials  were  originally  drawn.  The  history  of  a 
living  organism,  then,  is  one  of  incessant  change;  and  the 
conditions  of  this  change  are  to  be  found  partly  in  the  organ- 
ism itself,  and  partly  in  the  external  agencies  to  which  it  is 
subjected.  That  condition  which  is  inherent  in  the  organism, 
being  derived  hereditarily  from  its  progenitors,  may  be  con- 
veniently termed  its  germinal  capacity ;  its  parallel  in  the  in- 
organic world  being  that  fundamental  difference  in  properties 
which  constitutes  the  distinction  between  one  substance, 
whether  elementary  or  compound,  and  another ;  in  virtue  of 
which  each  "  behaves"  in  its  own  characteristic  manner  when 
subjected  to  new  conditions. 

Thus,  although  there  may  be  nothing  in  the  aspect  or  sen- 
sible properties  of  the  germ  of  a  Polype,  to  distinguish  from 
that  of  a  Man,  we  find  that  each  develops  itself,  if  the  requi- 
site conditions  be  supplied,  into  its  typical  form,  and  no  other; 
if  the  developmental  conditions  required  by  either  be  not  sup- 
plied we  do  not  find  a  different  type  evolved,  but  no  evolution 
at  all  takes  place.* 

*  It  is  quite  true  that  among  certain  of  the  lower  tribes,  both  of  PJants 


408      COBKELATION   OF   PHYSICAL  AND  VITAL   FOECE8. 

Now  the  difference  between  a  being  of  high  and  a  being 
of  low  organization  essentially  consists  in  this :  that  in  the 
latter  the  constituent  parts  of  the  fabric  evolved  by  the  pro- 
cess of  growth  from  the  original  germ,  are  similar  to  each 
other  in  structure  and  endowments,  whilst  in  the  former  they 
are  progressively  differentiated  with  the  advance  of  develop- 
ment, so  that  the  fabric  comes  at  last  to  consist  of  a  number 
of  organs,  or  instruments,  more  or  less/  dissimilar  in  struc- 
ture, composition,  and  endowments.  Thus  in  the  lowest 
forms  of  Vegetable  life,  the  primordial  germ  multiplies  itself 
by  duplicative  subdivision  into  an  apparently  unlimited  num- 
ber of  cells,  each  of  them  similar  to  every  other,  and  capa- 
ble of  maintaining  its  existence  independently  of  them.  And 
in  that  lowest  Bhizopod  type  of  Animal  life,  the  knowledge 
of  which  is  among  the  most  remarkable  fruits  of  modern  bio- 
logical research,  "  the  Physiologist  has  a  case  in  which  those 
vital  operations  which  he  is  elsewhere  accustomed  to  see  car- 
ried on  by  an  elaborate  apparatus,  are  performed  without  any 
special  instruments  whatever ;  a  little  particle  of  apparently 
homogeneous  jelly  changing  itself  into  a  greater  variety  of 
forms  than  the  fabled  Proteus,  laying  hold  of  its  food  without 
members,  swallowing  it  without  a  mouth,  digesting  it  without 
a  stomach,  appropriating  its  nutritious  material  without  absorb- 
ent vessels  or  a  circulating  system,  moving  from  place  to  place 
without  muscles,  feeling  (if  it  has  any  power  to  do  so)  with- 
out nerves,  propagating  itself  without  genital  apparatus,  and 

and  Animals,  especially  the  Fungi  and  Entozoa — similar  germs  may  devel- 
op themselves  into  very  dissimilar  forms,  according  to  the  conditions  un- 
der which  they  are  evolved ;  but  such  diversities  are  only  the  same  kind  as 
those  which  manifest  themselves  among  individuals  in  the  higher  Plants  and 
Animals,  and  only  show  that  in  the  types  in  question  there  is  a  less  close 
conformity  to  one  pattern.  Neither  in  these  groups,  nor  in  that  'group  of 
Foraminifera,  in  which  I  have  been  led  to  regard  the  range  of  variation 
as  peculiarly  great,  does  any  tendency  ever  show  itself  to  the  assumption 
of  the  characters  of  any  group  fundamentally  dissimilar. 


THE   LOWEST   GKADE   OF   LIFE.  409 

not  only  this,  but  in  many  instances  forming  shelly  coverings 
of  a  symmetry  and  complexity  not  surpassed  by  those  of  any 
testaceous  animals/'*  whilst  the  mere  separation  of  a  frag- 
ment of  this  jelly  is  sufficient  to  originate  a  new  and  inde- 
pendent organism,  so  that  any  number  of  these  beings  may 
be  produced  by  the  successive  detachment  of  such  particles 
from  a  single  Rhizopod,  each  of  them  retaining  (so  far  as  we 
have  at  present  the  means  of  knowing)  the  characteristic  en- 
dowments of  the  stock  from  which  it  was  an  offset. 

When,  on  the  other  hand,  we  watch  the  evolution  of  any 
of  the  higher  types  of  Organization,  whether  vegetable  or 
animal,  we  observe  that  although  in  the  first  instance  the  pri- 
mordial cell  multiplies  itself  by  duplicative  subdivision  into 
an  aggregation  of  cells,  which  are  apparently  but  repetitions 
of  itself  and  of  each  other,  this  homogeneous  extension  has 
in  each  case  a  definite  limit,  speedily  giving  place  to  a  struc- 
tural differentiation,  which  becomes  more  and  more  decided 
with  the  progress  of  development,  until  in  that  most  hetero- 
geneous of  all  types — the  Human  Organism — no  two  parts  are 
precisely  identical,  except  those  which  correspond  to  each 
other  on  the  opposite  sides  of  the  body.  With  this  structural 
differentiation  is  associated  a  corresponding  differentiation  of 
function ;  for  whilst  in  the  life  of  the  most  highly  developed 
and  complex  organism  we  witness  no  act  which  is  not  fore- 
shadowed, however  vaguely,  in  that  of  the  lowest  and  sim- 
plest, yet  we  observe  in  it  that  same  "  division  of  labour" 
which  constitutes  the  essential  characteristic  of  the  highest 
grade  of  civilization.  For,  in  what  may  be  termed  the  ele- 
mentary form  of  Human  Society,  in  which  every  individual 
relies  upon  himself  alone  for  the  supply  of  all  his  wants,  no 
greater  result  can  be  obtained  by  the  aggregate  action  of  the 
entire  community  than  its  mere  maintenance ;  but  as  each  in- 

*  See  the  Author's  Introduction  to  the  Study  of  the  Foraminifera,  pub- 
lished by  the  Ray  Society,  1862  :  Preface,  p.  vii. 
18 


410      CORRELATION   OF  PHYSICAL   AND  VITAL  FORCES. 

dividual  selects  a  special  mode  of  activity  for  himself,  and 
aims  at  improvement  in  that  specialty,  he  finds  himself  attain- 
ing a  higher  and  still  higher  degree  of  aptitude  for  it ;  and 
this  specialization  tends  to  increase  as  opportunities  arise  for 
new  modes  of  activity,  until  that  complex  fabric  is  evolved 
which  constitutes  the  most  developed  form  of  the  Social  State 
wherein  every  individual  finds  the  work — mental  or  bodily — 
for  which  he  is  best  fitted,  and  in  which  he  may  reach  the 
highest  attainable  perfection ;  while  the  mutual  dependence 
of  the  whole  (which  is  the  necessary  result  of  this  specializa- 
tion of  parts)  is  such  that  every  individual  works  for  the 
benefit  of  all  his  fellows,  as  well  as  for  his  own.  As  it  is 
only  in  such  a  state  of  society  that  the  greatest  triumphs  of 
human  ability  become  possible,  so  is  it  only  in  the  most  dif- 
ferentiated types  of  Organization  that  Vital  Activity  can  pre- 
sent its  highest  manifestations.  In  the  one  case,  as  in  the 
other,  does  the  result  depend  upon  a  process  of  gradual  devel- 
opment, in  which,  under  the  influence  of  agencies  whose  na- 
ture constitutes  a  proper  object  of  scientific  inquiry,  that  most 
general  form  in  which  the  fabric — whether  corporeal  or  social 
— originates,  evolves  itself  into  that  most  special  in  which  its 
development  culminates. 

But  notwithstanding  the  wonderful  diversity  of  structure 
and  of  endowments  which  we  meet  with  in  the  study  of  any 
complex  Organism,  we  encounter  a  harmonious  unity  or  coor- 
dination in  its  entire  aggregate  of  actions,  which  is  yet  more 
wonderful.  It  is  in  this  harmony  or  coordination,  whose  ten- 
dency is  to  the  conservation  of  the  organism,  that  the  state  of 
Health  or  Normal  life  essentially  consists.  And  the  more 
profound  our  investigations  of  its  conditions,  the  more  definite 
becomes  the  conclusion  to  which  we  are  led  by  the  study  of 
them — that  it  is  fundamentally  based  on  the  common  origin 
of  all  these  diversified  parts  in  the  same  germ,  the  vital  en- 
dowments of  which,  equally  diffused  throughout  the  entire 
fabric  in  those  lowest  forms  of  organization  in  which  every 


THE  FUNDAMENTAL   CHARACTERISTIC   OF  LIFE.       411 

part  is  but  a  repetition  of  every  other,  are  differentiated  in 
the  highest  amongst  a  variety  of  organs,  acquiring  in  virtue 
of  this  differentiation  a  much  greater  intensity. 

Thus,  then,  we  may  take  that  mode  of  Vital  Activity 
which  manifests  itself  in  the  evolution  of  the  germ  into  the 
complete  organism  repeating  the  type  of  its  parent,  and  the 
subsequent  maintenance  of  that  organism  in  its  integrity,  in 
the  one  case  as  in  the  other,  at  the  expense  of  materials  de- 
rived from  external  sources — as  the  most  universal  and  most 
fundamental  characteristic  of  Life  ;  and  we  have  now  to  con- 
sider the  nature  and  source  of  the  Force  or  Power  by  which 
that  evolution  is  brought  about.  The  prevalent  opinion  has 
until  lately  been,  that  this  power  is  inherent  in  the  germ ; 
which  has  been  supposed  to  derive  from  its  parent  not  merely 
its  material  substance,  but  a  nisus  formativus,  bildungstrieb, 
or  germ  force,  in  virtue  of  which  it  builds  itself  up  into  the 
likeness  of  its  parent,  and  maintains  itself  in  that  likeness 
until  the  force  is  exhausted,  at  the  same  time  imparting  a 
fraction  of  it  to  each  of  its  progeny.  In  this  mode  of  view- 
ing the  subject,  all  the  organizing  force  required  to  build  up 
an  Oak  or  a  Palm,  an  Elephant  or  a  Whale,  must  be  concen- 
trated in  a  minute  particle,  only  discernible  by  microscopic 
aid,  and  the  aggregate  of  all  the  germ-forces  appertaining  to 
the  descendants,  however  numerous,  of  a  common  parentage, 
must  have  existed  in  their  original  progenitors.  Thus  in  the 
case  of  the  successive  viviparous  broods  of  Aphides,  a  germ- 
force  capable  of  organizing  a  mass  of  living,  structure,  which 
would  amount  (it  has  been  calculated)*  in  the  tenth  brood 
to  the  bulk  of  five  hundred  millions  of  stout  men,  must  have 
been  shut  up  in  the  single  individual,  weighing  perhaps  the 
l-1000th  of  a  grain,  from  which  the  first  brood  was  evolved. 
And  in  like  manner,  the  germ-force  which  has  organized  the 

*  See  Prof.  Huxley  on  the  "  Agamic  Reproduction  of  Aphis,"  in  Linn. 
Trans.,  vol.  xxii.,  p.  215. 


412      CORRELATION   OF   PHYSICAL  AND  VITAL   FORCES. 

bodies  of  all  the  individual  men  that  have  lived  from  Adam 
to  the  present  day,  must  have  been  concentrated  in  the  body 
of  their  common  ancestor.  A  more  complete  reductio  ad  db- 
surdum  can  scarcely  be  brought  against  any  hypothesis  ;  and 
we  may  consider  it  proved  that  in  some  way  or  other,  fresh 
organizing  force  is  constantly  being  supplied  from  without 
during  the  whole  period  of  the  exercise  of  its  activity. 

When  we  look  carefully  into  the  question,  however,  we 
find  that  what  the  germ  really  supplies  is  not  the  force,  but 
the  directive  agency  ;  thus  rather  resembling  the  control  exer- 
cised by  the  superintendent  builder,  who  is  charged  with 
working  out  the  design  of  the  architect,  than  the  bodily  force 
of  the  workmen  who  labour  under  his  guidance  in  the  con- 
struction of  the  fabric.  The  actual  constructive  force,  as  we 
learn  from  an  extensive  survey  of  the  phenomena  of  life,  is 
supplied  by  Heat,  the  influence  of  which  upon  the  rate  of 
growth  and  development,  both  animal  and  vegetable,  is  so 
marked  as  to  have  universally  attracted  the  attention  of  Phys- 
iologists, who,  however,  have  for  the  most  part  only  recognized 
in  it  a  vital  stimulus  that  calls  forth  the  latent  power  of  the 
germ,  instead  of  looking  upon  it  as  itself  furnishing  the  power 
that  does  the  work.  It  has  been  from  the  narrow  limitation 
of  the  area  over  which  physiological  research  has  been  com- 
monly prosecuted,  that  the  intimacy  of  this  relationship  be- 
tween Heat  and  the  Organizing  force  has  not  sooner  become 
apparent.  Whilst  the  vital  phenomena  of  Warm-blooded  ani- 
mals, which  possess  within  themselves  the  means  of  main- 
taining a  constant  temperature,  were  made  the  sole,  or  at  any 
rate  the  chief  objects  of  study,  it  was  not  likely  that  the  in- 
quirer would  recognize  the  full  influence  of  external  heat  in 
accelerating,  or  of  cold  in  retarding  their  functional  activity. 
It  is  only  when  the  survey  is  extended  to  Cold-blooded  ani- 
mals and  to  Plants,  that  the  immediate  and  direct  relation  be- 
tween Heat  and  Vital  Activity,  as  manifested  in  the  rate  of 
growth  and  development,  or  of  other  changes  peculiar  to  the 


DYNAMICS   OF   GEEMINATION.  4:13 

living  body,  is  unmistakably  manifested.  To  some  of  those 
phenomena,  which  afford  the  best  illustrations  of  the  mode 
in  which  Heat  acts  upon  the  living  organism,  attention  will 
now  be  directed. 

Commencing  with  the  Vegetable  kingdom,  we  find  that  the 
operation  of  Heat  as  the  motive  power  or  dynamical  agency, 
to  which  the  phenomena  of  growth  and  development  are  to  be 
referred,  is  peculiarly  well  seen  in  the  process  of  Germina- 
tion. The  seed  consists  of  an  embryo  which  has  already  ad- 
vanced to  a  certain  stage  of  development,  and  of  a  store  of 
nutriment  laid  up  as  the  material  for  its  further  evolution ; 
and  in  the  fact  that  this  evolution  is  carried  on  at  the  expense 
of  organic  compounds  already  prepared  by  extrinsic  agency, 
until  (the  store  of  these  being  exhausted)  the  young  plant  is 
sufficiently  far  advanced  in  its  development  to  be  able  to  elab- 
orate them  for  itself,  the  condition  of  the  germinating  embryo 
resembles  that  of  an  Animal.  Now,  the  seed  may  remain 
(under  favourable  circumstances)  in  a  state  of  absolute  inac- 
tion during  an  unlimited  period.  If  secluded  from  the  free 
access  of  air  and  moisture,  and  kept  at  a  low  temperature,  it 
is  removed  from  all  influences  that  would  on  the  one  hand 
occasion  its  disintegration,  or  on  the  other,  would  call  it  into 
active  life.  But  when  again  exposed  to  air  and  moisture,  and 
subjected  to  a  higher  temperature,  it  either  germinates  or  de- 
cays, according  as  the  embryo  it  contains  has  or  has  not  pre- 
served its  vital  endowments — a  question  which  only  experi- 
ment can  resolve.  The  process  of  germination  is  by  no  means 
a  simple  one.  The  nutriment  stored  up  in  the  seed  is  in  great 
part  in  the  condition  of  insoluble  starch ;  and  this  must  be 
brought  into  a  soluble  form  before  it  can  be  appropriated  by 
the  embryo.  The  metamorphosis  is  effected  by  the  agency  of 
a  ferment  termed  diastase,  which  is  laid  up  in  the  immediate 
neighbourhood  of  the  embryo,  and  which,  when  brought  to 
act  on  starch,  converts  it  in  the  first  instance  into  soluble  dex- 
trine, and  then  (if  its  action  be  continued)  into  sugar.  The 


414      CORRELATION   OF  PHYSICAL   AND  VITAL  FORCES. 

dextrine  and  sugar,  combined  with  the  albuminous  and  oily 
compounds  also  stored  up  in  the  seed,  form  the  "protoplasm," 
which  is  the  substance  immediately  supplied  to  the  young 
plant  as  the  material  of  its  tissues  ;  and  the  conversion  of  this 
protoplasm  into  various  forms  of  organized  tissue,  which  be- 
come more  and  more  differentiated  as  development  advances, 
is  obviously  referable  to  the  vital  activity  of  the  germ.  Now 
it  can  be  very  easily  shown  experimentally  that  the  rate  of 
growth  in  the  germinating  embryo  is  so  closely  related  (within 
certain  limits)  to  the  amount  of  heat  supplied,  as  to  place  its 
dependence  on  that  agency  beyond  reasonable  question :  so 
that  we  seem  fully  entitled  to  say  that  Heat,  acting  through 
the  germ,  supplies  the  constructive  force  or  power  by  which  the 
Vegetable  fabric  is  built  up.*  But  there  appears  to  be  another 
source  of  that  power  in  the  seed  itself.  In  the  conversion  of 
the  insoluble  starch  of  the  seed  into  sugar,  and  probably  also 
in  a  further  metamorphosis  of  a  part  of  that  sugar,  a  large 
quantity  of  carbon  is  eliminate'd  from  the  seed  by  combining 
with  the  oxygen  of  the  air,  so  as  to  form  carbonic  acid ;  this 
combination  is  necessarily  attended  with  a  disengagement  of 
heat,  which  becomes  very  sensible  when  (as  in  molting)  a 
large  number  of  germinating  seeds  are  aggregated  together ; 
and  it  cannot  but  be  regarded  as  probable  that  the  heat  thus 
evolved  within  the  seed  concurs  with  that  derived  from  with- 
out, in  supplying  to  the  germ  the  force  that  promotes  its  evo- 
lution. 

*  The  effect  of  Heat  is  doubtless  manifested  very  differently  by  differ- 
ent seeds ;  such  variations  being  partly  specific,  partly  individual.  But 
these  are  no  greater  than  we  see  in  the  inorganic  world :  the  increment  of 
temperature  and  the  augmentation  of  bulk  exhibited  by  different  substances 
when  subjected  to  the  same  absolute  measure  of  heat,  being  as  diverse  as 
the  substances  themselves.  The  whole  process  of  "  Malting,"  it  may  be 
remarked,  is  based  on  the  regularity  with  which  the  seeds  of  a  particular 
species  may  be  at  any  time  forced  to  a  definite  rate  of  germination  by  a 
definite  increment  of  temperature. 


DYNAMIC   FUNCTIONS   OF   PLANTS.  415 

The  condition  of  the  Plant  which  has  attained  a  more  ad- 
vanced stage  of  its  development,  differs  from  that  of  the  ger- 
minating embryo  essentially  in  this  particular,  that  the  organic 
compounds  which  it  requires  as  the  materials  of  the  extension 
of  the  fabric  are  formed  by  itself,  instead  of  being  supplied  to 
it  from  without.  The  tissues  of  the  coloured  surfaces  of  the 
leaves  and  stems,  when  acted  on  by  light,  have  the  peculiar 
power  of  generating — at  the  expense  of  carbonic  acid,  water, 
and  ammonia — various  ternary  and  quaternary  organic  com- 
pounds, such  as  chlorophyll,  starch,  oil,  and  albumen ;  and  of 
the  compounds  thus  generated,  some  are  appropriated  by  the 
constructive  force  of  the  plant  (derived  from  the  heat  with 
which  it  is  supplied)  to  the  formation  of  new  tissues  ;  whilst 
others  are  stored  up  in  the  cavities  of  those  tissues,  where 
they  ultimately  serve  either  for  the  evolution  of  parts  subse- 
quently developed,  or  for  the  nutrition  of  animals  which  em- 
ploy .them  as  food.  Of  the  source  of  those  peculiar  affinities 
by  which  the  components  of  the  starch,  albumen,  &c.,  are 
brought  together,  we  have  no  right  to  speak  confidently ;  but 
looking  to  the  fact  that  these  compounds  are  not  produced  in 
any  case  by  the  direct  union  of  their  elements,  and  that  a  de- 
composition of  binary  compounds  seems  to  be  a  necessary 
antecedent  of  their  formation,  it  is  scarcely  improbable  that, 
as  suggested  by  Prof.  Le  Conte  (op.  cit.),  that  source  is  to  be 
found  in  the  chemical  forces  set  free  in  the  preliminary  act  of 
decomposition,  in  which  the  elements  would  be  liberated  in 
that  "  nascent  condition"  which  is  well  known  to  be  one  of 
peculiar  energy. 

The  influence  of  Light,  then,  upon  Vegetable  organism  ap- 
pears to  be  essentially  exerted  in  bringing  about  what  may  be 
considered  a  higher  mode  of  chemical  combination  between 
oxygen,  hydrogen,  and  carbon,  with  the  addition  of  nitrogen 
in  certain  cases  ;  and  there  is  no  evidence  that  it  extends  be- 
yond this.  That  the  appropriation  of  the  materials  thus  pre- 
pared, and  their  conversion  into  organized  tissue  in  the  opera- 


416      COEEELATION    OF   PHYSICAL   AJCD   VITAL   FOECES. 

tions  of  growth  and  development,  are  dependent  on  the  agency 
of  Heat,  is  just  as  evident  in  the  stage  of  maturity  as  in  that 
of  germination.  And  there  is  reason  to  believe,  further,  that 
an  additional  source  of  organizing  force  is  to  be  found  in  the 
retrograde  metamorphosis  of  organic  compounds  that  goes  on 
during  the  whole  life  of  the  plant ;  of  which  metamorphosis 
the  expression  is  furnished  by  the  production  of  carbonic  acid. 
This  is  peculiarly  remarkable  in  the  case  of  the  Fungi,  which, 
being  incapable  of  forming  new  compounds  under  the  influ- 
ence of  light,  are  entirely  supported  by  the  organic  matters 
they  absord,  and  which  in  this  respect  correspond  on  the  one 
hand  with  the  germinating  embryo,  and  on  the  other  with 
Animals.  Such  a  decomposition  of  a  portion  of  the  absorbed 
material  is  the  only  conceivable  source  of  the  large  quantity 
of  carbonic  acid  they  are  constantly  giving  out ;  and  it  would 
not  seem  unlikely  that  the  force  supplied  by  this  retrograde 
metamorphosis  of  the  superfluous  components  of  their  food, 
which  fall  down  (so  to  speak)  from  the  elevated  plane  of  "prox- 
imate principles,"  to  the  lower  level  of  comparatively  simple 
binary  compounds,  supplies  a  force  which  raises  another  por- 
tion to  the  rank  of  living  tissue,  thus  accounting  in  some  de- 
gree for  the  very  rapid  growth  for  which  this  tribe  of  Plants 
is  so  remarkable.  This  exhalation  of  carbonic  acid,  however, 
is  not  peculiar  to  Fungi  and  germinating  embryos,  for  it  takes 
place  during  the  whole  life  of  flowering  plants,  both  by  day 
and  by  night,  in  sunshine  and  in  shade,  and  from  their  green 
as  well  as  from  their  dark  surfaces  ;  and  it  is  not  improbable 
that,  as  in  the  case  of  the  Fungi,  its  source  lies  partly  in  the 
organic  matters  absolved,  recent  investigations  *  having  ren- 
dered it  probable  that  Plants  really  take  up  and  assimilate 
soluble  humus,  which,  being  a  more  highly  carbonized  sub- 
stance than  starch,  dextrine,  or  cellulose,  can  only  be  con- 

*  See  the  Memoir  of  M.  Risler,  "  On  the  Absorption  of  Humus,"  in 
the  "Bibliotheque  UniverseUe,"  N.  S.,  1858,  torn.  L,  p.  305. 


PLAUT-PRODUCTS   STORES   OF  FORCE.  417 

verted  into  compounds  of  the  latter  kind  by  parting  with  some 
of  its  carbon.  But  it  may  also  take  place  at  the  expense  of 
compounds  previously  generated  by  the  plant  itself,  and  stored 
up  in  its  tissues  ;  of  which  we  seem  to  have  an  example  in 
the  unusual  production  of  carbonic  acid  which  takes  place  at 
the  period  of  flowering,  especially  in  such  plants  as  have  a 
fleshy  disk,  or  receptacle  containing  a  large  quantity  of  starch  ; 
and  thus,  it  may  be  surmised,  an  extra  supply  of  force  is  pro- 
vided for  the  maturation  of  those  generative  products  whose 
preparation  seems  to  be  the  highest  expression  of  the  vital 
power  of  the  vegetable  organism. 

The  entire  aggregate  of  organic  compounds  contained  in 
the  vegetable  tissues,  then,  may  be  considered  as  the  expres- 
sion, not  merely  of  a  certain  amount  of  the  material  elements, 
oxygen,  hydrogen,  carbon,  and  nitrogen,  derived  (directly  or 
indirectly)  from  the  water,  carbonic  acid,  and  ammonia  of  the 
atmosphere,  but  also  of  a  certain  amount  of  force  which  has 
been  exerted  in  raising  these  from  the  lower  plane  of  simple 
binary  compounds,  to  the  higher  level  of  complex,  "  proxi- 
mate principles  ;"  whilst  the  portion  of  these  actually  converted 
into  organized  tissue  may  be  considered  as  the  expression  of 
a  further  measure  of  force,  which,  acting  under  the  directive 
agency  of  the  germ,  has  served  to  build  up  the  fabric  in  its 
characteristic  type.  This  constructive  action  goes  on  during 
the  whole  Life  of  the  Plant,  which  essentially  manifests  itself 
either  in  the  extension  of  the  original  fabric  (to  which  in 
many  instances  there  seems  no  determinate  limit),  or  in  the 
production  of  the  germs  of  new  and  independent  organisms. 
It  is  interesting  to  remark  that  the  development  of  the  more 
permanent  parts  involves  the  successional  decay  and  renewal 
of  parts  whose  existence  is  temporary.  The  "  fall  of  the 
leaf"  is  the  effect,  not  the  cause,  of  the  cessation  of  that  pe- 
culiar functional  activity  of  its  tissues,  which  consists  in  the 
elaboration  of  the  nutritive  material  required  for  the  produc- 
tion of  wood.  And  it  would  seem  as  if  the  duration  of  their 
18* 


418     CORRELATION  OP  PHYSICAL  AND  VITAL  FORCES. 

existence  stands  in  an  inverse  ratio  to  the  energy  of  their  ac- 
tion ;  the  leaves  of  "evergreens,"  which  are  not  cast  off  until 
the  appearance  of  a  new  succession,  effecting  their  functional 
changes  at  a  much  less  rapid  rate  than  do  those  of  "  deci- 
duous "  trees,  whose  term  of  life  is  far  more  brief. 

Thus,  the  final  cause  or  purpose  of  the  whole  Vital  Activ- 
ity of  the  Plant,  so  far  as  the  individual  is  concerned,  is  to 
produce  an  indefinite  extension  of  the  dense,  woody,  almost 
inert,  but  permanent  portions  of  the  fabric,  by  the  successional 
development,  decay,  and  renewal  of  the  soft,  active,  and  tran- 
sitory cellular  parenchyma ;  and  according  to  the  principles 
already  stated,  the  descent  of  a  portion  of  the  materials  of  the 
latter  to  the  condition  of  binary  compounds,  which  is  mani- 
fested in  the  largely-increased  exhalation  of  carbonic  acid 
that  takes  place  from  the  leaves  in  the  later  part  of  the  sea- 
son, comes  to  the  aid  of  external  Heat  in  supplying  the  force 
by  which  another  portion  of  those  materials  is  raised  to  the 
condition  of  organized  tissue.  The  vital  activity  of  the 
Plant,  however,  is  further  manifested  in  the  provision  made 
for  the  propagation  of  its  race,  by  the  production  of  the 
germs  of  new  individuals  ;  and  here,  again,  we  observe  that 
whilst  a  higher  temperature  is  usually  required  for  the  devel- 
opment of  the  flower,  and  the  maturation  of  the  seed,  than 
that  which  suffices  to  sustain  the  ordinary  processes  of  vege- 
tation, a  special  provision  appears  to  be  made  in  some  in- 
stances for  the  evolution  of  force  in  the  sexual  apparatus  it- 
self, by  the  retrograde  metamorphosis  of  a  portion  of  the  or- 
ganic compounds  prepared  by  the  previous  nutritive  opera- 
tions. This  seems  the  nearest  approach  presented  in  the 
Vegetable  organism,  to  what  we  shall  find  to  be  an  ordinary 
mode  of  activity  in  the  Animal.  That  the  performance  of 
the  generative  act  involves  an  extraordinary  expenditure  of 
vital  force  appears  from  this  remarkable  fact,  that  blossoms 
which  wither  and  die  as  soon  as  the  ovules  have  been  fertil- 
ized, may  be  kept  fresh  for  a  long  period  if  fertilization  be 
prevented. 


RECONVERSION  OF  ORGANIC   FORCES.  419 

The  decay  which  is  continually  going  on  during  the  life 
of  a  Plant  restores  to  the  inorganic  world,  in  the  form  of  car- 
bonic acid,  water,  and  ammonia,  a  part  of  the  materials  drawn 
from  it  in  the  act  of  vegetation  ;  and  a  reservation  being  made 
of  those  vegetable  products  which  are  consumed  as  food  by 
Animals,  or  which  are  preserved  (like  timber,  flax,  cotton, 
&c.)  in  a  state  of  permanence,  the  various  forms  of  decom- 
position which  take  place  after  death  complete  that  restora- 
tion. But  in  returning,  however  slowly,  to  the  condition  of 
water,  carbonic  acid,  ammonia,  &c.,  the  constituents  of 
Plants  give  forth  an  amount  of  heat  equivalent  to  that  which 
they  would  generate  by  the  process  of  ordinary  combustion  ; 
and  thus  they  restore  to  the  inorganic  world,  not  only  the  ma- 
terials, but  the  forces,  at  the  expense  of  which  the  vegetable 
fabric  was  constructed.  It  is  for  the  most  part  only  in  the 
humblest  Plants,  and  in  a  particular  phase  of  their  lives,  that 
such  a  restoration  takes  place  in  the  form  of  motion,  this  mo- 
tion being,  like  growth  and  development,  an  expression  of  the 
vital  activity  of  the  "Zoospores"  of  Algo3,  and  being  ob- 
viously intended  for  their  dispersion. 

Hence  we  seem  justified  in  affirming  that  the  Correlation 
between  heat  and  the  organizing  force  of  Plants  is  not  less 
intimate  than  that  which  exists  between  heat  and  motion. 
The  special  attribute  of  the  vegetable  germ  is  its  power  of 
utilizing,  after  its  own  particular  fashion,  the  heat  which  it 
receives,  and  of  applying  it  as  a  constructive  power  to  the 
building-up  of  its  fabric  after  its  characteristic  type. 


420      CORRELATION   OF  PHYSICAL   AND  VITAL  FORCES. 


II.— RELATIONS  OF  LIGHT  AND  HEAT   TO  THE  VITAL  FORCES 
OF  ANIMALS. 

THOSE  of  our  readers  who  accompanied  us  through  the 
first  part  of  our  inquiry  are  aware  that  it  was  our  object  to 
show,  that  as  force  is  never  lost  in  the  inorganic  world,  so 
force  is  never  created  in  the  organic ;  but  that  those  various 
operations  of  vegetable  life  which  are  sometimes  vaguely- 
attributed  to  the  agency  of  an  occult  "  vital  principle,"  and 
are  referred  by  more  exact  thinkers  to  certain  Vital  Forces 
inherent  in  the  organism  of  the  plant,  are  really  sustained  by 
solar  light  and  heat.  These,  we  have  argued,  supply  to 
each  germ  the  whole  power  by  which  it  builds  itself  up,  at  the 
expense  of  the  materials  it  draws  from  the  Inorganic  Uni- 
verse, into  the  complete  organism ;  while  the  mode  in  which 
that  power  is  exerted  (generally  as  vital  force,  specially  as 
the  determining  cause  of  the  form  peculiar  to  each  type)  de- 
pends upon  the  "germinal  capacity"  or  directive  agency  inher- 
ent in  each  particular  germ.  The  first  stage  in  this  construc- 
tive operation  consists  in  the  production  of  certain  organic 
compounds  of  a  purely  chemical  nature — such  as  gum,  starch, 
sugar,  chlorophyll,  oil,  and  albumen — at  the  expense  of  the 
oxygen,  hydrogen,  carbon,  and  nitrogen  derived  from  the 
water,  carbonic  acid,  and  ammonia  of  the  atmosphere ; 
whilst  the  second  consists  in  the  further  elevation  of  a  portion 
of  these  organic  compounds  to  the  rank  of  organized  tissue 
possessing  attributes  distinctively  vital.  Of  the  whole  amount 
of  organic  compounds  generated  by  the  plant,  it  is  but  a 
comparatively  small  part  (a)  that  undergoes  this  progressive 
metamorphosis  into  living  tissue.  Another  small  proportion 
(b)  undergoes  a  retrograde  metamorphosis,  by  which  the  orig- 
inal binary  components  are  reproduced ;  and  in  this  descent 
of  organic  compounds  to  the  lower  plane,  the  power  con- 


GRADES   OF   ORGANIC   ASCENT.  421 

sumed  in  their  elevation  is  given  forth  in  the  form  of  heat 
and  organizing  force  (as  is  specially  seen  in  germination), 
which  help  to  raise  the  portion  a  to  a  higher  level.  But  by 
far  the  larger  part  (c)  of  the  organic  compounds  generated 
by  plants  remains  stored  up  in  their  fabric,  without  undergo- 
ing any  further  elevation  ;  and  it  is  at  the  expense  of  these, 
rather  than  of  the  actual  tissues  of  plants,  that  the  life  of 
animals  is  sustained. 

When,  instead  of  yielding  up  any  portion  of  its  substance 
for  the  sustenance  of  animals,  the  entire  vegetable  organism 
undergoes  retrograde  metamorphosis,  it  not  only  gives  back 
to  the  inorganic  world  the  binary  compounds  from  which  it 
derived  its  own  constituents,  but  in  the  descent  of  the  several 
components  of  its  fabric  to  that  simple  condition — whether  by 
ordinary  combustion  (as  in  the  burning  of  coal)  or  by  slow 
decay — it  gives  out  the  equivalents  of  the  light  and  heat  by 
which  they  were  elevated  in  the  first  instance. 

In  applying  these  views  to  the  interpretation  of  the  phe 
nomena  of  animal  life,  we  find  ourselves,  at  the  commence- 
ment of  our  inquiry,  on  a  higher  platform  (so  to  speak)  than 
that  from  which  we  had  to  ascend  in  watching  the  construc- 
tive processes  of  the  plant.  For,  whilst  the  plant  had  first 
to  prepare  the  pabulum  for  its  developmental  operations,  the 
animal  has  this  already  provided  for  it,  not  only  at  the  ear- 
liest phase  of  its  development,  but  during  the  whole  period 
of  its  existence ;  and  all  its  manifestations  of  vital  activity 
are  dependent  upon  a  constant  and  adequate  supply  of  the 
same  pabulum.  The  first  of  these  manifestations  is,  as  in  the 
plant,  the  building-up  of  the  organism  by  the  appropriation 
of  material  supplied  from  external  sources  under  the  directive 
agency  of  the  germ.  The  ovum  of  the  animal,  like  the  seed 
of  the  plant,  contains  a  store  of  appropriate  nutriment  pre- 
viously elabprated  by  the  parent ;  and  this  store  suffices  for 
the  development  of  the  embryo,  up  to  the  period  at  which  it 
can  obtain  and  digest  alimentary  materials  for  itself.  That 


422     COEBELATION   OF  PHYSICAL   AND   VITAL   FOECES. 

period  occurs,  in  the  different  tribes  of  animals,  at  very  dis- 
similar stages  of  the  entire  developmental  process.  In  many 
of  the  lower  classes,  the  embryo  comes  forth  from  the  egg, 
and  commences  its  independent  existence,  in  a  condition 
which,  as  compared  with  the  adult  form,  would  be  as  if  a 
human  embryo  were  to  be  thrown  upon  the  world  to  obtain 
its  own  subsistence  only  a  few  weeks  after  conception  ;  and  its 
whole  subsequent  growth  and  development  takes  place  at  the 
expense  of  the  nutriment  which  it  ingests  for  itself. 

We  have  examples  of  this  in  the  class  of  insects,  many  of 
which  come  forth  from  the  egg  in  the  state  of  extremely  simple 
and  minute  worms,  having  scarcely  any  power  of  movement, 
but  an  extraordinary  voracity.  The  eggs  having  been  depos- 
ited in  situations  Kitted  to  afford  an  ample  supply  of  appropriate 
nutriment  (those  of  the  flesh-fly,  for  example,  being  laid  in 
carcases,  and  those  of  the  cabbage-butterfly  upon  a  cabbage- 
leaf)  ,  each  larva  on  its  emersion  is  as  well  provided  with  ali- 
mentary material  as  if  it  had  been  furnished  with  a  large  sup- 
plemental yolk  of  its  own  ;  and  by  availing  itself  of  this,  it 
speedily  grows  to  many  hundred  or  even  many  thousand 
times  its  original  size,  without  making  any  considerable  ad- 
vance in  development.  But  having  thus  laid  up  in  its  tissues  a 
large  additional  store  of  material,  it  passes  into  a  state  which, 
so  far  as  the  external  manifestations  of  life  are  concerned, 
is  one  of  torpor,  but  which  is  really  one  of  great  develop- 
mental activity  :  for  it  is  during  the  pupa  state  that  those  new 
parts  are  evolved,  which  are  characteristic  of  the  perfect  in- 
sect, and  of  which  scarcely  a  trace  was  discoverable  in  the 
larva  ;  so  that  the  assumption  of  this  state  may  be  likened  in 
many  respects  to  a  reentrance  of  the  larva  into  the  ovum. 
On  its  termination,  the  imago  or  perfect  insect  comes  forth 
complete  in  all  its  parts,  and  soon  manifests  the  locomotive 
and  sensorial  powers  by  which  it  is  specially  distinguished, 
and  of  which  the  extraordinary  predominance  seems  to  jus- 
tify our  regarding  insects  as  the  types  of  purely  animal  life. 


DEVELOPMENT  OF  INSECTS.  423 

There  are  some  insects  whose  imago-life  has  but  a  very  short 
duration,  the  performance  of  the  generative  act  being  appar- 
ently the  only  object  of  this  state  of  their  existence :  and  such 
for  the  most  part  take  no  food  whatever  after  their  final  emer- 
sion, their  vital  activity  being  maintained,  for  the  short  period 
it  endures,  by  the  material  assimilated  during  their  larva 
state.*  But  those  whose  period  of  activity  is  prolonged,  and 
upon  whose  energy  there  are  extraordinary  demands,  are 
scarcely  less  voracious  in  their  imago  than  in  their  larva- 
condition  ;  the  food  they  consume  not  being  applied  to  the 
increase  of  their  bodies,  which  grow  very  little  after  the  as- 
sumption of  the  imago-state,  but  chiefly  to  their  maintenance  ; 
no  inconsiderable  portion  of  it,  however,  being  appropriated 
in  the  female  to  the  production  of  ova,  the  entire  mass  of 
which  deposited  by  a  single  individual  is  sometimes  enormous. 
That  the  performance  of  the  generative  act  involves  not 
merely  a  consumption  of  material,  but  a  special  expenditure 
of  force,  appears  from  a  fact  to  be  presently  stated,  corre- 
sponding to  that  already  noticed  in  regard  to  plants. 

Now  if  we  look  for  the  source  of  the  various  forms  of 
vital  force — which  may  be  distinguished  as  constructive, 
sensori-motor,  and  generative — that  are  manifested  in  the  dif- 
ferent stages  of  the  life  of  an  insect,  we  find  them  to  lie,  on 
the  one  hand,  in  the  heat  with  which  the  organism  is  sup- 
plied from  external  sources,  and,  on  the  other,  in  the  food 
provided  for  it.  The  agency  of  heat,  as  the  moving  power 
of  the  constructive  operations,  is  even  more  distinctly  shown 
in  the  development  of  the  larva  within  the  egg,  and  in  the 
development  of  the  imago  within  its  pupa-case,  than  it  is  in 

*  It  is  not  a  little  curious  that  in  the  tribe  of  Rotifera,  or  Wheel-ani- 
malcules, all  the  males  yet  discovered  are  entirely  destitute  of  digestive 
apparatus,  and  are  thus  incapable  of  taking  any  food  whatever ;  so  that  not 
only  the  whole  of  their  development  within  the  egg,  but  the  whole  of  their 
active  life  after  their  emersion  from  it,  is  carried  on  at  the  expense  of  the 
store  of  yolk  provided  by  the  parent. 


424:     CORRELATION   OF  PHYSICAL   AND   VITAL  FORCES. 

the  germinating  seed ;  the  rate  of  each  of  these  processes 
being  strictly  regulated  by  the  temperature  to  which  the 
organism  is  subjected.  Thus  ova  which  are  ordinarily  not 
hatched  until  the  leaves  suitable  for  the  food  of  their  larvse 
have  been  put  forth,  may  be  made,  by  artificial  heat,  to  pro- 
duce a  brood  in  the  winter ;  whilst,  on  the  other  hand,  if  they 
be  kept  at  a  low  temperature,  their  hatching  may  be  retarded 
almost  indefinitely  without  the  destruction  of  their  vitality. 
The  same  is  true  of  the  pupa-state  ;  and  it  it  remarkable  that 
during  the  latter  part  of  that  state,  in  which  the  developmental 
process  goes  on  with  extraordinary  rapidity,  there  is  in  cer- 
tain insects  a  special  provision  for  an  elevation  of  the  tem- 
perature of  the  embryo  by  a  process  resembling  incubation. 
Whether,  in  addition  to  the  heat  imparted  from  without,  there 
is  any  addition  of  force  developed  within  (as  in  the  germinat- 
ing seed)  by  the  return  of  a  part  of  the  organic  constituents 
of  the  food  to  the  condition  of  binary  compounds,  cannot  at 
present  be  stated  with  confidence :  the  probability  is,  how- 
ever, that  such  a  retrograde  metamorphosis  does  take  place, 
adequate  evidence  of  its  occurrence  during  the  incubation  of 
the  bird's  egg  being  afforded  by  the  liberation  of  carbonic 
acid,  which  is  there  found  to  be  an  essential  condition  of  the 
developmental  process.  During  the  larva-state  there  is  very 
little  power  of  maintaining  an  independent  temperature,  so 
that  the  sustenance  of  vital  activity  is  still  mainly  due  to 
the  heat  supplied  from  without.  But  in  the  active  state  of  the 
perfect  insect  there  is  a  production  of  heat  quite  comparable 
to  that  of  warm-blooded  animals  ;  and  this  is  effected  by  the 
retrograde  metamorphosis  of  certain  organic  constituents  of 
the  food,  of  which  we  find  the  expression  in  the  exhalation  of 
carbonic  acid  and  water.  Thus  the  food  of  animals  becomes 
an  internal  source  of  heat,  which  may  render  them  independ- 
ent of  external  temperature. 

Further,  a  like  retrograde  metamorphosis  of  certain  con- 
stituents of  the  food  is  the  source  of  that  sensori-motor  power 


VITAL   FORCES      OF   INSECTS.  425 

which  is  the  peculiar  characteristic  of  the  animal  organism ; 
for  on  the  one  hand  the  demand  for  food,  on  the  other  the 
amount  of  metamorphosis  indicated  by  the  quantity  of  car- 
bonic acid  exhaled,  bear  a  very  close  relation  to  the  quantity 
of  that  power  which  is  put  forth.  This  relation  is  peculiarly 
manifest  in  insects,  since  their  conditions  of  activity  and  re- 
pose present  a  greater  contrast  in  their  respective  rates  of 
metamorphosis,  than  do  those  of  any  other  animals.  Of  the 
exercise  of  generative  force  we  have  no  similar  measure  ;  but 
that  it  is  only  a  special  modification  of  ordinary  vital  activity 
appears  from  this  circumstance,  that  the  life  of  those  insects 
which  ordinarily  die  very  soon  after  sexual  congress  and  the 
deposition  of  the  ova,  may  be  considerably  prolonged  if  the 
sexes  be  kept  apart  so  that  congress  cannot  take  place. 
Moreover,  it  has  been  shown  by  recent  inquiries  into  the 
agamic  reproduction  of  insects  and  other  animals,  that  the 
process  of  generation  differs  far  less  from  those  reproductive 
acts  which  must  be  referred  to  the  category  of  the  ordinary 
nutritive  processes,  than  had  been  previously  supposed. 

Thus,  then,  we  find  that  in  the  animal  organism  the  de- 
mand for  food  has  reference  not  merely  to  its  use  as  a  mate- 
rial for  the  construction  of  the  fabric ;  food  serves  also  as  a 
generator  of  force ;  and  this  force  may  be  of  various  kinds — 
heat  and  motor-power  being  the  principal  but  by  no  means 
the  only  modes  under  which  it  manifests  itself.  We  shall 
now  inquire  what  there  is  peculiar  in  the  sources  of  the  vital 
force  which  animates  the  organisms  of  the  higher  animals  at 
different  stages  of  life. 

That  the  developmental  force  which  occasions  the  evolu- 
tion of  the  germ  in  the  higher  vertebrata  is  really  supplied 
by  the  heat  to  which  the  ovum  is  subjected,  may  be  regarded 
as  a  fact  established  beyond  all  question.  In  frogs  and 
other  amphibia,  which  have  no  special  means  of  imparting  a 
high  temperature  to  their  eggs,  the  rate  of  development  (which 
in  the  early  stages  can  be  readily  determined  with  great  exact- 


426      CORRELATION   OF   PHYSICAL   AND   VITAL   FORCES. 

ness)  is  entirely  governed  by  the  degree  of  warmth  to  which 
the  ovum  is  subjected.  But  in  serpents  there  is  a  peculiar 
provision  for  supplying  heat ;  the  female  performing  a  kind 
of  incubation  upon  her  eggs,  and  generating  in  her  own  body 
a  temperature  much  above  that  of  the  surrounding  air.*  In 
birds,  the  developmental  process  can  only  be  maintained  by 
the  steady  application  of  external  warmth,  and  this  to  a  de- 
gree much  higher  than  that  which  is  needed  in  the  case  of 
cold-blooded  animals  ;  and  we  may  notice  two  results  of  this 
application  as  very  significant  of  the  dynamical  relation  be- 
tween heat  and  developmental  force — first,  that  the  period 
required  for  the  evolution  of  the  germ  into  the  mature  embryo 
is  nearly  constant,  each  species  having  a  definite  period  of 
incubation — and  second,  that  the  grade  of  development  at- 
tained by  the  embryo  before  its  emersion  is  relatively  much 
higher  than  it  is  in  cold-blooded  vertebrata  generally;  the 
only  instances  in  which  any  thing  like  the  same  stage  is  at- 
tained without  a  special  incubation,  being  those  in  which  (as 
in  the  turtle  and  crocodile)  the  eggs  are  hatched  under  the 
influence  of  a  high  external  temperature.  This  higher  devel- 
opment is  attained  at  the  expense  of  a  much  greater  consump- 
tion of  nutrient  material ;  the  store  laid  up  in  the  "  food  yolk" 
and  "  albumen"  of  the  bird's  egg,  being  many  tunes  greater 
in  proportion  to  the  size  of  the  animal  which  laid  it,  than  that 
contained  in  the  whole  egg  of  a  frog  or  a  fish.  There  is 
evidence  in  that  liberation  of  carbonic  acid  which  has  been 
ascertained  to  go  on  in  the  egg  (as  in  the  germinating  seed) 
during  the  whole  of  the  developmental  process,  that  the  return 
of  a  portion  of  the  organic  substances  provided  for  the  suste- 

*  In  the  Viper  the  eggs  are  usually  retained  within  the  oviduct  until 
they  are  hatched.  In  the  Python,  which  recently  went  through  the  process 
of  incubation  in  the  Zoological  Gardens,  the  eggs  were  imbedded  in  the 
coils  of  the  body ;  the  temperature  to  which  they  were  subjected  (as  ascer- 
tained by  a  thermometer  placed  in  the  midst  of  them)  averaging  90°  F., 
whilst  that  of  the  cage  averaged  60°  F. 


DYNAMICS   OF   EMBKYONIC   DELELOPMENT.  427 

nance  of  the  embryo,  to  the  condition  of  simple  binary  com- 
pounds, is  an  essential  condition  of  the  process ;  and  since  it 
can  scarcely  be  supposed  that  the  object  of  this  metamor- 
phosis can  be  to  furnish  heat  (an  ample  supply  of  that  force 
being  afforded  by  the  body  of  the  parent),  it  seems  not  un- 
likely that  its  purpose  is  to  supply  a  force  that  concurs  with 
the  heat  received  from  without  in  maintaining  the  process  of 
organization. 

The  development  of  the  embryo  within  the  body,  in  the 
mammalia,  imparts  to  it  a  steady  temperature  equivalent  to 
that  of  the  parent  itself;  and  in  all  save  the  implacental  or- 
ders of  this  class,  that  development  is  carried  still  further 
than  in  birds,  the  new-born  mammal  being  yet  more  com- 
plete in  all  its  parts,  and  its  size  bearing  a  larger  proportion 
to  that  of  its  parent,  than  even  in  birds.  It  is  doubtless  ow- 
ing in  great  part  to  the  constancy  of  the  temperature  to  which 
the  embryo  is  subjected,  that  its  rate  of  development  (as 
shown  by  the  fixed  term  of  utero-gestation)  is  so  uniform. 
The  supply  of  organizable  material  here  afforded  by  the  ovum 
itself  is  very  small,  and  suffices  only  for  the  very  earliest 
stage  of  the  constructive  process ;  but  a  special  provision  is 
very  soon  made  for  the  nutrition  of  the  embryo  by  materials 
directly  supplied  by  the  parent ;  and  the  imbibition  of  these 
takes  the  place,  during  the  whole  remainder  of  foetal  life,  of 
the  appropriation  of  the  materials  supplied  in  the  bird's  egg 
by  the  "  food  yolk"  and  "  albumen."  To  what  extent  a  retro- 
grade metamorphosis  of  nutrient  material  takes  place  in  the 
foetal  mammal,  we  have  no  precise  means  of  determining; 
since  the  products  of  that  metamorphosis  are  probably  for  the 
most  part  imparted  (through  the  placental  circulation)  to  the 
blood  of  the  mother,  and  got  rid  of  through  her  excretory 
apparatus.  But  sufficient  evidence  of  such  a  metamorphosis 
is  afforded  by  the  presence  of  urea  in  the  amniotic  fluid  and 
of  biliary  matter  in  the  intestines,  to  make  it  probable  that  it 
takes  place  not  less  actively  (to  say  the  least)  in  the  fetal 


428      CORKELATION   OF   PHYSICAL    AND   VITAL   FOECES. 

mammal  than  it  does  in  the  chick  in  ovo.  Indeed,  it  is  im- 
possible to  study  the  growth  of  any  of  the  higher  organisms — 
which  not  merely  consists  in  the  formation  of  new  parts,  but 
also  involves  a  vast  amount  of  interstitial  change — without 
perceiving  that  in  the  remodelling  which  is  incessantly  going 
on,  the  parts  first  formed  must  be  removed  to  make  way  for 
those  which  have  to  take  their  place.  And  such  removal  can 
scarcely  be  accomplished  without  a  retrograde  metamorphosis, 
which,  as  in  the  numerous  cases  already  referred  to,  may  be 
considered  with  great  probability  as  setting  free  constructive 
force  to  be  applied  in  the  production  of  new  tissue. 

If,  now,  we  pass  on  from  the  intra-uterine  life  of  the 
mammalian  organism  to  that  period  of  its  existence  which 
intervenes  between  birth  and  maturity,  we  see  that  a  tempo- 
rary provision  is  made  in  the  acts  of  lactation  and  nursing 
for  affording  both  food  and  warmth  to  the  young  creature, 
which  is  at  first  incapable  of  adequately  providing  itself  with 
aliment,  or  of  resisting  external  cold  without  fostering  aid. 
And  we  notice  that  the  offspring  of  man  remains  longer  de- 
pendent upon  parental  care  than  that  of  any  other  mammal, 
in  accordance  with  the  higher  grade  of  development  to  be 
ultimately  attained.  But  when  the  period  of  infancy  has 
passed,  the  child  is  adequately  supplied  with  food,  and  is  pro- 
tected by  the  clothing  which  makes  up  for  the  deficiency  of 
other  tegumentary  covering,  ought  to  be  able  to  maintain  its 
own  heat,  save  in  an  extremely  depressed  temperature ;  and 
this  it  does  by  the  metamorphosis  of  organic  substances,  partly 
derived  from  its  own  fabric,  and  partly  supplied  directly  by 
the  food,  into  binary  compounds.  During  the  whole  period 
of  growth  and  development,  we  find  the  producing  power  at 
its  highest  point ;  the  circulation  of  blood  being  more  rapid, 
and  the  amount  of  carbonic  acid  generated  and  thrown  off 
being  much  greater  in  proportion  to  the  bulk  of  the  body, 
than  at  any  subsequent  period  of  life.  We  find,  too,  in  the 
large  amount  of  other  excretions,  the  evidence  of  a  rapid 


CONSTRUCTIVE  FOECES  OF   ANIMALS.  429 

metamorphosis  of  tissue  ;  and  it  can  hardly  be  questioned  (if 
our  general  doctrines  be  well  founded)  that  the  constructive 
force  that  operates  in  the  completion  of  the  fabric  will  be  de- 
rived in  part  from  the  heat  so  largely  generated  by  chemical 
change,  and  in  part  from  the  descent  which  a  portion  of  the 
fabric  itself  is  continually  making  from  the  higher  plane  of 
organized  tissue  to  the  lower  plane  of  dead  matter.  This 
high  measure  of  vital  activity  can  only  be  sustained  by  an 
ample  supply  of  food ;  which  thus  supplies  both  material  for 
the  construction  of  the  organism,  and  the  force  by  whose 
agency  that  construction  is  accomplished. 

How  completely  dependent  the  constructive  process  still  is 
upon  heat,  is  shown  by  the  phenomena  of  reparation  in  cold- 
blooded animals ;  since  not  only  can  the  rate  at  which  they 
take  place  be  experimentally  shown  to  bear  a  direct  relation 
to  the  temperature  to  which  these  animals  are  subjected,  but 
it  has  been  ascertained  that  any  extraordinary  act  of  repara- 
tion (such  as  the  reproduction  of  a  limb  in  the  salamander) 
will  only  be  performed  under  the  influence  of  a  temperature 
much  higher  than  that  required  for  the  maintenance  of  the 
ordinary  vital  activity.  After  the  maturity  of  the  organism 
has  been  attained,  there  is  no  longer  any  call  for  a  larger 
measure  of  constructive  force  than  is  required  for  the  mainten- 
ance of  its  integrity ;  but  there  seems  evidence  that  even  then 
the  required  force  has  to  be  supplied  by  a  retrograde  meta- 
morphosis of  a  portion  of  the  constituents  of  the  food,  over  and 
above  that  which  serves  to  generate  animal  heat.  For  it  has 
been  experimentally  found  that,  in  the  ordinary  life  of  an  adult 
mammal,  the  quantity  of  food  necessary  to  keep  the  body  in 
its  normal  condition  is  nearly  twice  that  which  would  be  re- 
quired to  supply  the  "  waste  "  of  the  organism,  as  measured 
by  the  total  amount  of  excreta  when  food  is  withheld ;  and 
hence  it  seems  almost  certain  that  the  descent  of  a  portion  of 
the  organic  constituents  of  this  food  to  the  lower  level  of  sim- 
ple binary  compounds  is  a  necessary  condition  of  the  ele- 


4:30      CORRELATION   OF  PHYSICAL   AND  VITAL   FORCES. 

vation  of  another  portion  to  the  state  of  living   organized 
tissue. 

The  conditions  of  animal  existence,  moreover,  involve  a 
constant  expenditure  of  motor  force  through  the  instrumental- 
ity of  the  nervo-muscular  apparatus  ;  and  the  exercise  of  the 
purely  psychical  powers,  through  the  instrumentality  of  the 
brain,  constitutes  a  further  expenditure  of  force,  even  when 
no  bodily  exertion  is  made  as  its  result.  We  have  now  to 
consider  the  conditions  under  which  these  forces  are  devel- 
oped, and  the  sources  from  which  they  are  derived. 

The  doctrine  at  present  commonly  received  among  physi- 
ologists upon  these  points  may  be  stated  as  follows : — The 
functional  activity  of  the  nervous  and  muscular  apparatuses 
involves,  as  its  necessary  condition,  the  disintegration  of  their 
tissues ;  the  components  of  which,  uniting  with  the  oxygen 
of  the  blood,  enter  into  new  and  simpler  combinations,  which 
are  ultimately  eliminated  from  the  body  by  the  excretory  oper- 
ations. In  such  a  retrograde  metamorphosis  of  tissue,  we 
have  two  sources  of  the  liberation  of  force  : — first,  its  descent 
from  the  condition  of  living,  to  that  of  dead  matter,  involving 
a  liberation  of  that  force  which  was  originally  concerned  in ' 
its  organization  ;* — and  second,  the  further  descent  of  its 
complex  organic  components  to  the  lower  plane  of  simple 
binary  compounds.  If  we  trace  back  these  forces  to  their 
proximate  source,  we  find  both  of  them  in  the  food  at  the 

*  It  was  by  Liebig  ("Animal  Chemistry,"  1842)  that  the  doctrine  was 
first  distinctly  promulgated  which  had  been  already  more  vaguely  affirmed 
by  various  Physiologists,  that  every  production  of  motion  by  an  Animal 
involves  a  proportional  disintegration  of  muscular  substance.  But  he  seems 
to  have  regarded  the  motor  force  produced  as  the  expression  only  of  the 
vital  force  by  which  the  tissue  was  previously  animated  ;  and  to  have  looked 
upon  its  disintegration  by  oxygenation  as  simply  a  consequence  of  its  death. 
The  doctrine  of  the  "  Correlation  of  Forces  "  being  at  that  time  undevel- 
oped, he  was  not  prepared  to  recognize  a  source  of  motor  power  in  the  ulte- 
rior chemical  changes  which  the  substance  of  the  muscle  undergoes ;  but 
seems  to  have  regarded  them  as  only  concerned  in  the  production  of  heat. 


TEST   OF  ANIMAL   MOTOR   FOECE.  431 

expense  of  which  the  animal  organism  is  constructed ;  for 
besides  supplying  the  material  of  the  tissues,  a  portion  of 
that  food  (as  already  shown)  becomes  the  source,  in  its  retro- 
grade metamorphosis,  of  the  production  of  the  heat  which 
supplies  the  constructive  power,  whilst  another  portion  may 
afford,  by  a  like  descent,  a  yet  more  direct  supply  of  organ- 
izing force.  And  thus  we  find  in  the  action  of  solar  light 
and  heat  upon  plants — whereby  they  are  enabled  not  merely 
to  extend  themselves  almost  without  limit,  but  also  to  accu- 
mulate in  their  substance  a  store  of  organic  compounds  for 
the  consumption  of  animals — the  ultimate  source  not  only  of 
the  materials  required  by  animals  for  their  nutrition,  but  also 
of  the  forces  of  various  kinds  which  these  exert. 

Recent  investigations  have  rendered  it  doubtful,  however, 
whether  the  doctrine  that  every  exertion  of  the  functional 
power  of  the  nervo-muscular  apparatus  involves  the  disinte- 
gration of  a  certain  equivalent  amount  of  tissue,  really  ex- 
presses the  whole  truth.  It  has  been  maintained,  on  the  basis 
of  carefully-conducted  experiments,  in  the  first  place,  that  the 
amount  of  work  done  by  an  animal  may  be  greater  than  can 
be  accounted  for  by  the  ultimate  metamorphosis  of  the  azo- 
tized  constituents  of  its  food,  their  mechanical  equivalent 
being  estimated  by  the  heat  producible  by  the  combustion  of 
the  carbon  and  oxygen  which  they  contain  ;*  and  secondly, 
that  whilst  there  is  not  a  constant  relation  (as  affirmed  by 
Liebig)  between  the  amount  of  motor  force  produced  and  the 
amount  of  disintegration  of  muscular  tissue  represented  by 
the  appearance  of  urea  in  the  urine,  such  a  constant  relation 
does  exist  between  the  development  of  motor  force  and  the 
increase  of  carbonic  acid  in  the  expired  air,  as  shows  that 
between  these  two  phenomena  there  is  a  most  intimate  rela- 

*  This  view  has  been  expressed  to  the  author  by  two  very  high  author- 
ities, Prof.  Helmholtz  and  Prof.  William  Thomson,  independently  of  each 
other,  as  an  almost  necessary  inference  from  the  data  furnished  by  the 
experiments  of  Dr.  Joule. 


432      CORRELATION   OF  PHYSICAL   AND   VITAL   FORCES. 

tionship.*  And  the  concurrence  of  these  independent  indica- 
tions seems  to  justify  the  inference  that  motor  force  may  be 
developed,  like  heat,  by  the  metamorphosis  of  constituents 
of  food  which  are  not  converted  into  living  tissue  ; — an  infer- 
ence which  so  fully  harmonizes  with  the  doctrine  of  the  direct 
convertibility  of  these  two  forces,  now  established  as  one  of 
the  surest  results  of  physical  investigation,  as  to  have  in  itself 
no  inherent  improbability.  Of  the  conditions  which  deter- 
mine the  generation  of  motor  force,  on  the  one  hand,  from  the 
disintegration  of  muscular  tissue,  on  the  other  from  the  meta- 
morphosis of  the  components  of  the  food,  nothing  definite  can 
at  present  be  stated  ;  but  we  seem  to  have  a  typical  example 
of  the  former  in  the  parturient  action  of  the  uterus,  whose 
muscular  substance,  built  up  for  this  one  effort,  forthwith 
undergoes  a  rapid  retrograde  metamorphosis ;  whilst  it  can 
scarcely  be  regarded  as  improbable  that  the  constant  activity 
of  the  heart  and  of  the  respiratory  muscles,  which  gives 
them  no  opportunity  of  renovation  by  rest,  is  sustained  not  so 
much  by  the  continual  renewal  of  their  substance  (of  which 
renewal  there  is  no  histological  evidence  whatever)  as  by  a 
metamorphosis  of  matters  external  to  themselves,  supplying 
a  force  which  is  manifested  through  their  instrumentality. 

To  sum  up :  The  life  of  man,  or  of  any  of  the  higher 
animals,  essentially  consists  in  the  manifestation  of  forces 
of  various  kinds,  of  which  the  organism  is  the  instrument ; 
and  these  forces  are  developed  by  the  retrograde  metamor- 
phosis of  the  organic  compounds  generated  by  the  instru- 
mentality of  the  plant,  whereby  they  ultimately  return  to  the 
simple  binary  forms  (water,  carbonic  acid,  and  ammonia), 
which  serve  as  the  essential  food  of  vegetables.  Of  these 
organic  compounds,  one  portion  (a)  is  converted  into  the 

*  On  these  last  points  reference  is  especially  made  to  the  recent  experi- 
ments of  Dr.  Edward  Smith. 


SUMMARY  OF  THE  ARGUMENT.          433 

substance  of  the  living  body,  by  a  constructive  force  which 
(in  so  far  as  it  is  not  supplied  by  the  direct  agency  of  external 
heat)  is  developed  by  the  retrograde  metamorphosis  of  another 
portion  (6)  of  the  food.  And  whilst  the  ultimate  descent  of 
the  first-named  portion  (a)  to  the  simple  condition  from  which 
it  was  originally  drawn,  becomes  one  source  of  the  peculiarly 
animal  powers — the  psychical  and  the  motor — exerted  by  the 
organism,  another  source  of  these  may  be  found  in  a  like 
metamorphosis  of  a  further  portion  (c)  of  the  food  which  has 
never  been  converted  into  living  tissue. 

Thus,  during  the  whole  life  of  the  animal,  the  organism 
is  restoring  to  the  world  around  both  the  materials  and  the 
forces  which  it  draws  from  it ;  and  after  its  death  this  resto- 
ration is  completed,  as  in  plants,  by  the  final  decomposition 
of  its  substance.  But  there  is  this  marked  contrast  between 
the  two  kingdoms  of  organic  nature  in  their  material  and 
dynamical  relations  to  the  inorganic  world — that  whilst  the 
vegetable  is  constantly  engaged  (so  to  speak)  in  raising  its 
component  materials  from  a  lower  plane  to  the  higher,  by 
means  of  the  power  which  it  draws  from  the  solar  rays,  the 
animal,  whilst  raising  one  portion  of  these  to  a  still  higher 
level  by  the  descent  of  another  portion  to  a  lower,  ultimately 
lets  down  the  whole  of  what  the  plant  had  raised ;  in  so 
doing,  however,  giving  back  to  the  universe,  in  the  form  of 
heat  and  motion,  the  equivalent  of  the  light  and  heat  which 
the  plant  had  taken  from  it. 


19 


INDEX. 


Air-gun,  action  of  the,  219,  220. 
Ancient  method  of  inquiry,  317. 
Ancients,  philosophic  aims  of  the,  13. 

—  their  mode  of  explaining  phenomena, 

103. 

Andrews,  Dr.,  135, 162. 
Animal  force,  derivation  of,  423. 

—  heat,  source  of,  324. 

—  nutrition,  421. 

—  system,  early  conception  of,  212. 
Anomaly  in  expansion  and  contraction,  52. 
Aphides,  multiplication  of,  411. 
Aristotle,  817. 

Astronomy,  tendency  of  its  progress,  xi. 

—  difficulties  of,  319. 

—  early  development  of,  320. 
Atmosphere,  limit  of,  136. 

—  pressure  of,  319. 

Atomic  theory,  objections  to,  164. 
Atoms,  question  of  existence  of,  347. 
Authority,  value  of,  11. 
Automatic  machines,  211. 


B 


Bacon,  Francis,  philosophy  of,  14. 

Becquerel,  M.,  102, 125. 

Beclard,  M.,  175. 

Bergmann  on  electrical  effects,  35. 

Bessel,  241. 

Brodie,  Sir  B.,  175. 

Buffon,  314. 


Causation,  Grove  on,  15. 
—  secondary,  18. 
Cause  and  effect,  251. 

simultaneity  of,  17. 

Cause,  nature  of,  402. 
Caoutchouc,  anomaly  of,  53. 


Carbonic  acid  the  index  of  animal  power, 

431. 
Carnot,  224. 

—  on  the  transfer  of  heat,  70. 
Carpenter,  Dr.,  xxxi.,  xxxiv.,  173. 

—  biographical  notice  of,  400. 
Catalysis,  6, 169. 

Chemical  action,  what  is  it  ?  153. 

a  cause  of  light,  162. 

produces  magnetism,  162. 

T the  source  of  mechanical  power,  SOT. 

—  affinity  as  a  cause  of  motion,  152. 
a  cause  of  heat,  159. 

—  force  converted  into  electrical,  154. 
Chemistry  of  plants,  395. 

Clarke,  Mr.  Latimer,  129, 181. 

Clausius,  Prof.,  xxviii.,  104,  228. 

Cohesive  attraction,  171. 

Cold  regarded  dynamically,  48. 

Colding,  Prof.,  224. 

Coleridge,  108. 

Collision,  effects  of,  29. 

Combustion,  key  to  the  phenomena  of,  321. 

Comets,  Mayer  on,  270. 

Compression,  heat  resulting  from,  32. 

Conservation  of  force,  importance  of  the 

problem,  356. 

Constancy  of  the  sun's  mass,  282. 
Constructive  action  of  plants,  417. 
Cooling  of  the  earth,  80. 
Cordier,  M.,  307,  313. 
Correlation  of  physical  forces,  meaning  of 

the  phrase,  3,  383. 

the  problem  to  be  solved,  189. 

difficulties  of  the  investigation, 

194. 

Cosmogony,  Grove  on,  81. 
Cotton  factory  as  a  distributor  'of  force,  403. 


Daguerre,  M.,  112. 
Dalton,  157, 159. 


INDEX. 


435 


Darkness  converted  into  light,  119. 

Davy,  Sir  H.,  xxvii.,  4, 184,  245. 

Day,  variation  in  length  of  the,  303. 

De  Candolle,  57. 

Decay  restores  heat  to  the  universe,  419. 

De  la  Eive,  57. 

Despritz,  M.,  76. 

Democritus,  127. 

Descartes,  xi. 

Desormes,  Clement,  75. 

Development  of  the  embryo ;  derivation 

of  its  force,  427. 
Diathermancy,  278. 
Differentiation  in  animal  development, 

409. 

Diminution  of  the  earth's  volume,  303. 
Discovery,  conditions  of,  104. 
Distinctions  among  the  forces  imperfect, 

178. 

Dove,  M.,  126. 
Dufour,  M.,  96. 
Dulong  and  Petit,  189. 
Dynamic  function  of  the  germ,  412. 


E 


Earth,  age  of,  245. 

—  form  of,  297. 

Earth's  interior  heat,  236,  300. 

proofs  of,  302. 

reasonableness  of,  303. 

Effect  of  electrical  discharge  upon  gases, 

93.     . 

Electrical  induction,  84. 
Electricity,  limitation  of  the  term,  35. 

—  from  caoutchouc,  35. 

—  the  link  among  the  other  forces,  37. 

—  as  initiating  other  forces,  83. 

—  what  is  it?  83. 

—  produces  chemical  decomposition,  84. 

—  atmospheric,  87. 

—  intensity  of,  89. 

—  effect  upon  the  terminals,  90. 

—  molecular  changes  of  conductors,  95. 

—  hypothesis  of  a  fluid  unnecessary,  98, 

101. 

—  animal,  99. 

—  in  what  it  consists,  101. 

—  initiates  motion,  104. 

—  and  heat,  106. 

—  and  light,  107. 

—  produced  by  blowpipe  flame,  155. 
Electro-chemical  equivalence,  394. 

equivalents  of  power,  373. 

Electro-magnetic  equivalence,  393. 
Emotions,  correlations  of,  xxxiv. 
Ethereal  medium,  123,  242,  271,  347. 
objections  to,  133. 

Equivalent,  meaning  applied  to  the  term, 
329. 

Ericsson,  Mr.,  78. 

Erinan  on  friction  of  homogeneous  sub- 
stances, 33. 

Essences,  the  search  for,  14. 

Euler,  Leonard,  123.     . 

Events  can  never  be  repeated,  193 

Expansion,  inequality  of,  54. 


Falling  bodies,  highest  velocity  of,  337. 
phenomena  of.  318. 

—  force,  253,  343. 

Faraday,  Dr.,  discoveries   of,   6,  85,  144, 

—  biographical  notice  of,  858. 
Favre,  M.,  371. 

Faye's  theory  of  comets,  41. 
Flower  figures  in  ice,  50. 
Fizeau,  M.,  129. 
Forbes,  Jas.  D.,  xix. 
Force,  indestructibility  of,  xli. 

—  use  of  the  doctrine,  xiii. 

—  importance  of  new  views  of,  xxx. 

—  Grove's  definition  of,  19. 

—  imparted  to  machines,  218. 

—  what  is  it?  251. 

—  definition  of,  335. 

—  meaning  of  the  term,  330,  379. 

—  living  and  dead,  333. 

—  animal,  derived  from  decomposition  of 

food,  432. 
Foucault,  M.,  129. 
Fourier,  M.,  309. 
Franklin,  Dr,,  xvii. 
Fresnel  on  heat  repellancy,  41. 
Friction,  definition  of,  30. 

—  producing  electricity,  33. 

—  of  homogeneous  substances,  83. 

—  heat  of,  389. 

G 

Gassiot,  Mr.,  59, 135. 
Gay  Lussac,  M.,  167. 
Germ-force,  411. 
Germination,  dynamics  of,  413. 
Generative  force  in  insects,  425. 
Gravity  implies  a  plenum,  368. 

—  in  relation  to  other  forces,  170,  253. 

—  is  it  a  force  ?  339,  340,  341. 

—  but  half  understood.  866. 

—  relation  to  conservation  of  force,  363. 
Grove,  aim  of  his  essay,  19. 

—  biographical  notice  of,  2. 

—  claims  of,  5. 

—  electrical  images  on  glass,  86. 

—  discoveries  on  the  relations  of  heat  to 

the  gases  of  water,  65. 
Grotthus,  163. 

Growth  in  the  mammalia,  428. 
Guillernin,  M.,  151. 


Heat,  material  hypothesis  of,  annihilated 
by  Eumford,  xxiii.,  xxiv. 

—  definition  of,  39. 

—  dynamical  effects  of,  40. 

—  latent,  41,  348. 

—  influence  of  structure  over  the    con- 

dition of,  57. 

—  produces  the  various  forces,  57. 

—  reflection  of,  61. 

—  converted  into  light,  63. 


436 


INDEX. 


Heat  (continued). 

—  chemical  and  magnetic  Influence  of,  64. 

—  as  producing  motive-power,  68. 

—  practical  application  of,  77. 

—  produced    by    chemical    combination, 

cause  of,  161. 

—  definition  of,  226. 

—  force  only  available  in  the  cooling  pro- 

cess, 228. 
—r  universal  law  of,  261. 

—  sources  of,  261. 

—  solar,  its  chemical  origin,  266. 

—  developed  by  friction,  277. 

—  upon  high  mountains,  282. 

—  lost  by  volcanoes,  310. 

—  lost  by  the  ocean,  311. 

—  and  motion,  convertibility  of,  823. 

—  and  work,  326. 

—  converted  into  organizing  force,  412. 
Helmholtz,  175. 

—  biographical  notice  of,  210. 

—  claims  of,  224. 
Henry,  Prof.,  xxviii.,  290. 
Herschel,  Sir  John,  119,  264. 
Herschel,  Sir  W.,  118, 136. 

—  his  hypothesis  of  the  sun's  action,  260. 
Hipparchus,  243. 

History  of  scientific  discovery,  xv. 
Holtzmann,  351. 
Homer,  M.,  312. 
Hume  on  causation,  16. 
Huxley,  Prof.,  411. 
Hydraulic  ram,  4. 


Indestructibility  of  matter,  xii. 
Inertia,  relation  to  forces,  369. 
Intellectual  and  physical  forces,  xxxv. 
Intensity  of  the  sun's  heat,  270. 
Interaction  of  natural  forces,  211. 
Insects,  development  of,  422. 


Joule,  Dr.  J.  P.,  xxiv.,  8, 33, 151,  224,  312. 


K 

Kant,  23. 

Karsten,  85. 

KirchofFs  researches,  62. 

Knoblauch,  M.,  56, 118. 

Knowledge,  slow  growth  of,  12. 


Laplace,  231,  242,  243,  291. 
Latent  heat,  41,  348. 
Lavoisier,  xii. 
Leconte,  Prof.,  xxviii. 
Liebig,  vi,  174, 175,  238,  430. 

—  biographical  notice  of,  386. 
Light,  polarization  of,  110. 

—  chemical  action  of,  111. 

—  affects  all  forms  of  matter,  115. 


Light  (continued). 

—  produces  the  other  forces,  116. 

—  influence  of  the  recipient  surface  upon, 

120. 

—  and  sound,  analogies  of,  126. 

—  and  molecular  changes,  130. 

—  a  motion  of  ordinary  matter,  133. 

—  its  loss  or  absorption,  189. 

—  function  of,  in  organization,  415. 

Life  of  the  higher  animals,  in  what  it  con- 
sists, 432. 
Living  force,  218. 
Littrow,  271. 


M 


Machines  compared  with  the  living  sys- 
tem, 79. 

—  driving  force  of,  214,  387. 
Muggi,  Dr.,  148. 

Magnetism  a  directive  force,  142. 

—  influence  of,  upon  light  and  heat,  145. 

—  as  an  initiating  force,  147. 

—  static  and  dynamic.  149. 
Magneto-electric  machines,  222. 
Marrion,  Mr.,  151. 

Masson,  M.,  135. 

Mathematics,  function  of,  in  investigation, 

377. 
Matter,  ultimate  structure  of  may  never 

be  known,  187. 

Matteucci,  79,  84,  97, 151, 172, 175. 
Mayer,  Dr.,  3. 

—  Tyndall's  estimate  of,  xxx. 

—  biographical  sketch  of,  250. 

—  physiological    commencement   of  hia 

inquiries,  324. 

—  secures   his    discovery   against    rival 

claimants,  224. 

—  various  references  to,  xxix.,  241,  327, 

350,  389,  465. 

Measure  of  the  sun's  heat,  264. 
Mechanical  problem,  the  modern,  212. 

—  force  and  heat,  262. 

—  origin  of  solar  heat,  276. 

—  equivalent  of  heat,  316,  391. 

applications  of,  353. 

Melloni,  researches  of,  59. 
Mental  and  vital  forces,  xxxii. 
Metamorphosis  retrograde  in  animals,  421. 
Meteorites,  270. 

—  heat  generated  by,  234. 
Metaphysics  in  science,  360. 

Modern  science,  alleged  materializing  ten- 
dency of,  xi. 

Molecules,  how  the  term  is  employed  by 
Grove,  89. 

Moon,  effect  of  collision  with  the  earth, 
304. 

Mongolfier,  M.,  4. 

Moral  forces,  correlation  of,  xxxvii. 

Morgan,  Mr.,  135. 

Morfchini,  117. 

Mossoti,  171. 

Motion,  as  an  affection  of  matter,  26. 

—  never  destroyed,  27. 

—  produces  heat,  28. 

—  produces  various  forces,  87. 


INDEX. 


437 


Motion  (continued). 

—  an  affection  of  matter,  186. 

—  vast  effects  of,  217. 
Mountain  tops,  heat  of,  282. 

N 

Nebular  hypothesis,  230. 
Nerve-power,  source  of,  430. 
New  views,  public  acceptance  of,  9. 
New  doctrine  of  forces,  how  far  accepted, 

xiv. 
Newton,  xiii.,  xvii!.,  241,  2T6,  282, 368,  378. 

—  speculations  on  light,  137. 
Niepce  de  St.  Victor,  M.,  125. 
Numbers  the  highest  aim  of  investigation, 

320. 


Oersted's  discoveries,  5. 
Optic  axis  of  crystals,  171. 
Organic  beings,  source  of  their  motions, 
237. 

—  kingdoms  of  nature,  relations  to  each 

other,  433. 

—  co-ordination,  410. 

—  correlations,  Grove  on,  172. 
Organization,  distinction  between  high  and 

low  grades  of,  408. 
Origin  of  the  sun's  heat,  276. 
Origin  of  terrestrial  power,  236, 840. 


Page,  Mr.,  151. 

Particles,  in  what  sense  the  term  is  used 
by  Grove,  39. 

Pasteur,  M.,  132. 

Peltier,  M.,  96. 

Perpetual  motion,  191,  192,  212,  219,  221, 
223,  388. 

Persistence  of  force,  xxxix. 

Photography,  chemistry  of,  111. 

Physical  science  of  the  present  distin- 
guished from  that  of  the  past,  402. 

Planetary  atmospheres,  137. 

—  motions,  267. 

—  system,  structure  of,  230. 
Planets,  temperatures  of,  121. 
Plants,  chemistry  of;  420. 
Plenum,  a  universal,  134. 
Pliicker,  171. 

Plurality  of  worlds,  122. 

Poisson,  M.,  81. 

Pouillet,  M.,  244,  264,  280. 

Powell,  Baden,  on  Newton's  rings,  41. 

Practical  arts  after  the  middle  ages,  211. 

Ptolemaic  system,  11. 


Qualities  of  matter,  transference  of,  15. 

E 

Kankine,  Mr.,  xxviii. 
Eegnault,  M.,  225. 


Eespiration  in  cold-blooded  animals,  429. 

Eoget,  Dr.,  4. 

Eotation  of  the  earth,  diminution  of  the, 

299. 

Eoyal  Institution,  foundation  of  the,  xxx. 
Eule  for  the  investigation  of  nature,  316, 

Eumford,  Count,  4. 

—  summary  of  his  claims,  xv* 

—  sketch  of,  xvii. 

—  researches  of,  xx.,  347. 


Seebeck,  Dr.,  118. 
Seguin,  M.,  4,  76. 
Sensations  of  heat  disturb  the  judgment, 

Sensations,  misleading,  174. 

Science,  tendency  of,  toward  immaterial- 
ism,  xii. 

'—  the  great  event  in  the  general  progress 
of,  xvi. 

—  the  true  scope  of,  xxxi. 

—  discovery  of  its  fundamental  principle, 

316. 

Simultaneous  discovery,  examples  of,  xv. 
Social  forces,  correlation  of,  xxxvi. 
Solar  radiation,  effects  of,  236. 
origin  of  organic  power,  240. 

—  heat,  amount  of,  243,  264. 

—  spots,  286. 

—  atmosphere,  288. 

—  radiation,  dynamical  effects  of,  403, 404. 
Sondhauss,  Mr.,  126. 

Sound  and  light,  259. 

Specific  heat,  dynamic  view  of,  53. 

Spectrum,  analysis  of,  62. 

Spencer,  Herbert,  on  the  persistence  of 

force,  xxxix. 

Steam  engine,  action  of,  220. 
Stephenson,  George,  115. 
Stewart,  Balfour,  «L 
Store  of  force  in  the  universe,  232. 
in  the  planetary  system  at  present, 

Structure    of  bodies   as   affecting   heat- 
motion,  55. 
Sun,  cooling  of,  265. 


Talbot,  Mr.,  112. 
Tension,  a  static  force,  22. 
Theories,  immature,  110. 

—  new,  how  they  are  to  be  judged,  196. 

—  the  test  of,  276. 
Theorizing,  what  is  it  ?  198. 
Thermography,  58. 

Thilorier's  experiment  on  carbonic  acid,  48. 

Thompson,  Prof.,  52,  227,  229. 

Tidal  wave,  241,  291. 

Time  as  an  element  of  dynamic  changes, 

—  an  element  in,  the  sequence  of  phe- 

nomena, 17. 

—  necessity  of  its  early  measurement,  320. 


438 


INDEX, 


Torricelli,  108. 

Transmutation  of  force  chemically  illus- 
trated, 372. 
Tyndall,  xxx.,  57, 


Units  of  heat  produced  in  combustion,  261. 


Vacuum,  not  yet  obtained,  134. 
Vital  and  physical  forces,  correlation  of, 
xxxii. 

—  effects,  crude  explanations  of,  401. 

—  activity,  characteristics  of,  407. 
of  plants,  purpose  of,  418» 

—  principle,  402,  420. 
Vis  viva,  218,  263. 
Volcanoes,  loss  of  heat  by,  310. 
Volta,  153. 

Voltaic  arc,  cause  of  light  of,  88. 


W 


"Wartmann,  Mr.,  146. 

Water,   expansion   of,  as   it  approaches 

freezing,  50. 

"Water-power,  force  produced  by,  215. 
Watt,  James,  75. 
Wortheim,  96, 150. 
Whewell,  Dr.,  xxvi.,  136. 
Wollaston,  Dr.,  136. 
Wilson,  Dr.,  136. 

Words,  misguiding  influence  of,  94. 
—  their  social  import,  180. 
Work  performed  by  chemical  force,  226. 
Wood,  Dr.,  54, 160. 


Young,  Dr.  Thomas,  xxvii.,  124, 125, 128, 
129, 138. 


Zodiacal  light,  272. 


THE  END. 


THE  UNIVERSITY  LIBRARY 

UNIVERSITY  OF  CALIFORNIA,  SANTA  CRUZ 

SCIENCE  LIBRARY 

This  book  is  due  on  the  last  DATE  stamped  below. 

To  renew  by  phone,  call  459-2050. 

Books  not  returned  or  renewed  within  14  days 

after  due  date  are  subject  to  billing. 


Series  2477 


QC73.Y67  Sci 


3  2106  00238 7709 


